William,
If you turn off the inverter/charger and the shore power, leave the battery switches on, and then measure the house and chassis battery voltages with a digital meter at the main battery terminals that have the red wire and black wire leads that go to the coach attached to them. Record the house and chassis battery bank voltages. They will not be exactly the same.
Then go to the large red and black leads at the inverter charger and measure the DC voltage there. It will very closely match the voltage that you recorded for one of the battery banks given there is no load or charging going on. The one that is very close or the same is the one that is connected to the charger inverter. This avoids any confusion from a signal tracer. If that happens to be the chassis battery bank plus lead then you need to exchange the main plus leads on the two banks that head to the coach.
Later Ed