Just a thought but friends of ours bought a camper a while back and had a very similar problem. Found out that it was wired with an easily over looked battery disconnect switch. Soon as that little unmarked bugger was flipped on everything was honky dory again. And everything worked fine while plugged in to shore power with the switch on or off. That is a fairly common thing with living quarters horse trailers and some TTs. Just an old lady's mind grasping at straws. Oh that it could be that simple for your situation.