The OP says his system works fine in a "Bench Test" mode.
Monitor and display connected with a short cable works fine in his Pick-up truck.
Let's keep it simple....take the EXACT same setup (don't switch cables) and connect it to 12v in the RV (maybe directly to the battery. If that does not work, you have a problem with your 12v source in the RV. Also, you have two 12V systems in the RV, try both of them.
It should work if you connect directly to the RV battery.
Then change one thing...connect to the 12v in the dash.
Keep going by only changing one thing at a time.
As Bob said, it is likely one of the two cables that go to the back camera housing.
2014 40QBH Phaeton DP Cummins 380HP ISL, Freightliner XC, 2014 Jeep Trailhawk TOAD
Blue Ox Aventa tow bar, SMI Air Force One -South Orange County Ca.