Check the 2 main fuses, probably 30 amps each, on the 12 v distribution board at your converter.
Check the ground wire from the house batteries to the frame. The place where it contacts the frame may be corroded.
Follow the hot wire from the house battery to see where it goes. Its first stop should be the "discontent" relay/solenoid.
Look to see if there were 2 main hot wires to the house batteries, one to the "Benny" and one to the rest of the house. The one to the rest of the house may have gotten left off accidentally.
Following the hot wire and testing for voltage as you go is the only real way to find where the voltage stops.