I am assuming that you are saying you have no-AC-to-DC power, but all of your AC systems operate properly. But, when you select your generator, it shuts down after it actually starts??
Well, with this being the case, I would suspect your COACH batteries. They provide power for the RV's Power Control Panel which detects if the generator is started and running and if this is so, it will switch the RV systems OFF of the Shore Power and onto the GENERATOR.
I suspect that when you connect your Converter alone to power, it is able to operate your DC powered circuitry, but with the COACH batteries being bad, they are loading your Converter down and without that 12-VDC available, so the Power Control Panel switches the RV circuitry back over to your Shore power.
How did you check your batteries? Was it under load? If not, batteries may show they have voltage, but they will not be able to support a real load.
So, Check those batteries and replace them if necessary.