Ian

Not sure what your experience is.  I have two Retevis radios within a meter of me right now, and while the RT76P shipped with a fairly serious firmware bug, they fixed that.  My experience with Retevis has been somewhere between generally and overwhelmingly positive…

Think you just sold me an antenna!  I want one of the red ones to match the car's paint…  Looks like it'll do the job.

I'm on my third.  Bought an MXT-90 at Wal-Mart on silly good sale, got an MXT-100 on eBay as a spare for fifteen bucks or so, and upgraded to the MXT-275 ASAP when it came out.  That is and remains my "daily driver" mobile radio and base station.  Eventually it's going to be semi-permanently vehicle mounted with an Australian-style RJ45 passthrough in the dash.  That's already bought and mounted, currently just struggling to find a minimally-invasive antenna solution right now.  
Best I can find is a $330 Sti-Co antenna.  😕
Currently planning on upgrading to the new second-generation units which can handle split tones and the new hand-mic with volume knob, but I'm in no hurry.  

I'm on my third.  Bought an MXT-90 at Wal-Mart on silly good sale, got an MXT-100 on eBay as a spare for fifteen bucks or so, and upgraded to the MXT-275 ASAP when it came out.  That is and remains my "daily driver" mobile radio and base station.  Eventually it's going to be semi-permanently vehicle mounted with an Australian-style RJ45 passthrough in the dash.  That's already bought and mounted, currently just struggling to find a minimally-invasive antenna solution right now.  
Best I can find is a $330 Sti-Co antenna.  😕
Currently planning on upgrading to the new second-generation units which can handle split tones and the new hand-mic with volume knob, but I'm in no hurry.  

Anyone else ever tried this?   I bought the transformer for Ryobi 18v system but all the major tool suppliers are available.   Used the suppled connector to hook the transformer to the battery adapter and installed power pole connections downstream.  
 
For testing purposes I hooked it up to a Wouxun KG-XS20G.  The supplied power was 12.5V after the step down.  The radio needs 13.8v +/- 15% so should be within range.  Per the power meter, in RX the radio pulled about 0.2A and TX it was under 5A.  
 
Transmission was fine and did not seem to cause any issues.  Left in standby for several hours and transmitted with fellow radio enthusiasts for 5-10 min.  
 
I will do more testing but it looks like this might be another acceptable power source.   
 
Thoughts?  

If they add or even replace MURS with UHF FRS/GMRS, I will buy these for sure. I do have stock in Garmin though, as I own a lot of their products, and have for a long time. The Rino series radios are awesome! I am not even scared off with the $2k price tag for the power switch and 10" display package. 
I like that they are making products like this. And OP, thank you for sharing the links.

Purely anecdotal, but I and a friend have tried MURS vs GMRS in high desert country (no buildings or big mountains, etc), and MURS wins at 2w vs GMRS at 5w.  He lives there year-around and had no idea what MURS was until I loaned him my Wouxun KG-805M (and a Smiley OEM antenna).  He said "Thanks, I'm buying two of them."

Let's make this question legal.  You have access to a MURS base station and a GMRS base station.  The GMRS unit is turned to low power, 2W.  Both radios feed identical coax lines, leading to a tower extending 20' above your chimney (close to the limit for a MURS base station).  Each is connected to a similar, functionally equivalent antenna of 3-5 dB gain.  
I still can't tell you without more specifics!  In terms of range, Height Is Might.  Now you have the same line-of-sight for both radios, and because of that, they'll both go approximately just as far.  Depending on your local foliage, the VHF might have an advantage -- pine needles really interfere with UHF frequencies, so if you have to punch through a stand of pine trees, the MURS will likely have the advantage, for now.  The other place it counterintuitively wins is with its poor building penetration.  If you're trying to communicate with someone outside the building, you're swimming upstream… but that means all that RF energy bounces around INSIDE the building, leading to crystal clear radio calls between two people inside of the same stainless steel BigBoxMart building.  
Ultimately, I'm giving the win to the GMRS unit, since you can flip the power from "low" at two watts to "high" at fifty, and suddenly you're loud and clear waaay out at the distances where MURS is noisy and scratchy and you're mostly hearing the local Wal-Mart and not the person you're trying to communicate with.  
If we're going to play the two-watt game, the real winner might be the new FM CB radios; it's not unheard of (but not legal for) people to communicate across the Atlantic on four watts, if they have a good antenna and good conditions.  150 MHz and 465 MHz are both line-of-sight bands, whereas down in HF around 25 MHz you can start to get some ground-wave effects as well as being able to bounce off the ionosphere.  Depending on sunspot conditions, this can increase your range __wildly.__  CB is in the region where it's also vaguely feasible to attempt near-vertical-incidence-skywave propagation, which again depends on near-perfect conditions, but covers the donut between simplex communication and sky-bounce propagation; this range donut is actually significantly important for deployed military forces.  America's current method is to use a repeater in the sky, be it drone, AWACS or satellite, but historically jeeps with skywave antennas were used in this role.  
A surprising contender might be 900 MHz ISM radios; they're limited to just one watt, but if you struggle with intelligibility more than signal strength due to your local terrain and foliage, they will remain perfectly legible right up until the moment you go below a critical signal-to-noise ratio and fall off the "digital cliff".  However, these radios can ping each other, so you can push a button and KNOW if you're in range, or not, with zero ambiguity.  900 MHz also has excellent building penetration, rather the opposite of VHF's problem.  Sometimes, multipath "interference" is your friend!

Let's make this question legal.  You have access to a MURS base station and a GMRS base station.  The GMRS unit is turned to low power, 2W.  Both radios feed identical coax lines, leading to a tower extending 20' above your chimney (close to the limit for a MURS base station).  Each is connected to a similar, functionally equivalent antenna of 3-5 dB gain.  
I still can't tell you without more specifics!  In terms of range, Height Is Might.  Now you have the same line-of-sight for both radios, and because of that, they'll both go approximately just as far.  Depending on your local foliage, the VHF might have an advantage -- pine needles really interfere with UHF frequencies, so if you have to punch through a stand of pine trees, the MURS will likely have the advantage, for now.  The other place it counterintuitively wins is with its poor building penetration.  If you're trying to communicate with someone outside the building, you're swimming upstream… but that means all that RF energy bounces around INSIDE the building, leading to crystal clear radio calls between two people inside of the same stainless steel BigBoxMart building.  
Ultimately, I'm giving the win to the GMRS unit, since you can flip the power from "low" at two watts to "high" at fifty, and suddenly you're loud and clear waaay out at the distances where MURS is noisy and scratchy and you're mostly hearing the local Wal-Mart and not the person you're trying to communicate with.  
If we're going to play the two-watt game, the real winner might be the new FM CB radios; it's not unheard of (but not legal for) people to communicate across the Atlantic on four watts, if they have a good antenna and good conditions.  150 MHz and 465 MHz are both line-of-sight bands, whereas down in HF around 25 MHz you can start to get some ground-wave effects as well as being able to bounce off the ionosphere.  Depending on sunspot conditions, this can increase your range __wildly.__  CB is in the region where it's also vaguely feasible to attempt near-vertical-incidence-skywave propagation, which again depends on near-perfect conditions, but covers the donut between simplex communication and sky-bounce propagation; this range donut is actually significantly important for deployed military forces.  America's current method is to use a repeater in the sky, be it drone, AWACS or satellite, but historically jeeps with skywave antennas were used in this role.  
A surprising contender might be 900 MHz ISM radios; they're limited to just one watt, but if you struggle with intelligibility more than signal strength due to your local terrain and foliage, they will remain perfectly legible right up until the moment you go below a critical signal-to-noise ratio and fall off the "digital cliff".  However, these radios can ping each other, so you can push a button and KNOW if you're in range, or not, with zero ambiguity.  900 MHz also has excellent building penetration, rather the opposite of VHF's problem.  Sometimes, multipath "interference" is your friend!

That is interesting. I never realized it was based on MURS. I have a Garmin Xumo XT for my bike, it's been great. I looked at the group ride radio but honestly, I just never ride in groups. Now that I know it is based on MURS, it opens up the ability to talk with others using that system. Good to know it isn't some closed "Garmin" system, but based on a free to use, unlicensed public frequency.
It sure is expensive, though. But, I would feel awfully cool if I had a MURS and a GMRS mobile rig on my motorcycle. I bet all the pretty girls would think I was all right at that point.

It appears that the original poster wants to use a MURS freq as an Operational Fixed link to remotely to remotely control another station.

Correct!
Cross band repeat is okay because it’s all within the Amateur Radio Service. As far as I can tell, Cross Service Repeat is not permitted for any of the Personal Radio Services. 
Definitely repeaters are prohibited in MURS.

There are a number of amateur radios that do what you are suggesting but using 2m and 70cm frequencies.  Guess what that feature is called?  "Cross-band repeat".

Something that forwards audio via RF is a repeater. The fact that you're crossing services is a regulatory problem, but there are technical issues also.
How exactly would it work?  Two MURS simplex radios connecting two full-duplex GMRS repeaters? From a technical standpoint (disregarding the rules temporarily) that's not possible.  The only way to connect duplex repeaters requires duplex communications.
Would they be working automatically? Are the two GMRS repeaters within range of each other?  If not, how are the MURS radios within range of each other?

Ouch.  Seriously, though, I collect "unicorns" -- things the FCC approved though they violate FCC rules.  FRS radios with detachable antennas, MURS multiservice radios, type-certified DMR FRS radios, weird shit.

Without getting into the debate on Part 90 vs 95 figured I'd list some basic Repeaters for UHF that would be good base units if your shopping for new.
Low Power
RT97 - List Price - $536 (Sale Price - $371.99) - https://www.retevis.com/rt97-portable-gmrs-repeater-mobile-repeater#A9150CX1
Midland MRX10 - List Price - $ 459.00 - https://midlandusa.com/products/midland-mxr10-repeater
Mid Power
Bridgecomm BCR-40U - List Price $1499.00 - https://www.bridgecomsystems.com/collections/amateur-radio-repeaters/products/bcr-40u
ICOM FR6300H - List Price $1819.00 - https://theantennafarm.com/shop-by-categories/radios/repeater-systems/uhf-repeaters/11680-icom-fr6300h-detail
Kenwood NXR-810K - List Price - $1770.00 - https://theantennafarm.com/shop-by-categories/radios/repeater-systems/uhf-repeaters/9640-kenwood-nxr-810k-detail
Motorola SLR5700 - List Price $ 3700.00 - https://theantennafarm.com/shop-by-categories/radios/repeater-systems/uhf-repeaters/9581-motorola-slr5700-detail
To be noted none of the mid power repeaters come with a duplexer. That will tac on another $300 and up depending on what you purchase. 
 
Granted there are many UHF part 90 repeaters used on the market that can be had for way less money than above but wanted to list new prices. If your searching online and someone is advertising a GR1225 on GMRS for $1000 I'd think twice. For mid power I have no experience with the Bridgecomm or the new NRX (however have many of the older Kenwood) units. Just wanted to put some info in one location. 
The low power units are nice because they basically work out of the box. I also realize the prices on the mid power units look high when your comparing to some of the CCR world but they will outlast pretty much all the home brewed stuff as they are built to do what they are. 
Some used older stuff that can be had reasonably are the Vertex VXR Series, Motorola GR1225, Motorola MTR2000, Motorola Quantar, Kenwood TKR850, Midland Base Tech II (Was a solid unit), ICOM FR4000 and the Motorola XPR series.

Yes, range will definitely help. In a previous life, I used VHF comms for oil spill cleanup operations, then UHF was for a "command net" which was slightly shorter range, not FRS/GMRS but there were many that brought their own radios that were FRS/GMRS (pre-2017 changes). 
I would only like to see FRS/GMRS compatible unit as it would augment what I already have, lots of Garmin Rino's. Thank you for sharing the links, I may have to save my pennies for more toys to use with my other toys.

Ultimately, my goal with a garage repeater is essentially a chunky base station radio with a pocket-sized "terminal".  But I live in a coverage gap between all the repeaters in the area.  When conditions are good, I can occasionally hear one ID.  95% of the time, I hear nothing from them; 100% of the time, I can't open them up.
 
I think there's a role for the garage repeater, but that role goes away if there's preexisting good coverage.

Ouch.  Seriously, though, I collect "unicorns" -- things the FCC approved though they violate FCC rules.  FRS radios with detachable antennas, MURS multiservice radios, type-certified DMR FRS radios, weird shit.

For as limited as they are, my BF-888s are still in use.

yah... About that......

I'm not so much a Midland hater per say, but I'd say I was a little disappointed with my MXT-275 as well. I didn't have any real problems with it until I realized it was stuck in narrow band, and that it could not do split tones (I had the pre-c port version). One of our best repeaters in my area used DCS split tones, so that was a bummer. As I became more involved with GMRS, I quickly outgrew the radio and wanted more features, so I sold it to a fellow jeeper that just wanted a radio for simplex coms. 

Yeah, it's due to the much higher insertion losses when their tiny duplexer is tuned for a 5 MHz offset, I believe.
I'm amazed they didn't check to see if the VHF unit was operating in anything like a usable band configuration, to be honest…

I have three of the RT10's. They do work. RX sensitivity pretty good. TX power is lower than my 6580's but it seems adequate for what I needed them for, use within repeater useful range. TX audio is ok, RX has some strange audio expander, like with M's "hear clear". Does not do "squelch tail elimination" when talking into one of my 927 Quantar repeaters. 
It is also DMR and does that very well. 
So for a sub-$100 radio its ok. I'd still make my primary portable a XPR6580 though. Mobiles I use 35 Watt MCS2000's and for repeaters, Motorola Quantar.
GeorgeC W2DB/WQYN409
Crowley, TX
 

IMO... I wouldn't let the narrowband only grant turn me away if it otherwise met my needs. The biggest issue of wideband vs narrowband is the equipment is mismatched (narrowband radio into a widband repeater, or vice versa). It would just come down to setting up the radios accordingly.
On another note, from what I saw on retevis' sitez the stock tuning for the original rt97 wants 10mhz offset in vhf or uhf...not even feasible in vhf (2m), being only 4mhz wide. It makes me wonder if the lower rated power on the gmrs model is to get it to sneak under the thresholds for certification (from some of the numbers we saw in the report), if it's accounting for signal loss tuning that tiny duplexer down to a 5mhz offset, or a bit of both.