The 91 is a PITA for sure.
Under the small hood on the drivers side is a black panel that one has to open,
once inside that panel there are two relays, one for chassis battery disconnect and
the other for house battery disconnect.
Each relay has two large cables each, jumper the heavy cables on each relay and
see if you have power.
If you get power then you either have a bad relay( common problem) or the control
line that turns on the relay has problems.
It has been a couple of years since I had one of these units and had the problem also.
1999 Trade Winds 7371 Cat 3126B w/current upgrades
1990 D 250 Dodge Ram Cummins Turbo