Is your distribution panel in the from of the coach, behind the credenza or couch?
I had a similar problem with my 77 with the panel there. The printed circuit boards that the fuses mount to crack and then things get weird. For testing purposes only I would suggest that you try hooking the battery directly to the fuse panel and bypassing the 12V+ cable connection. Then connect the ground wire to a frame ground. If things work on battery only then you know you have a main fuse issue. Either the ground side or positive side is bad. You can get repplacement 50 AMP Fuse holders at the autoparts store but they will be for the blade type fuses and need to be connected in. Good luck and be safe as you test, 12 volt
systems can be more dangerous than a 120 volt one!