I think your problem is different.
Sounds like someone rewired to run the house off the engine battery. Most RVs keep them separated except for charging purposes.
Personally, I would rewire it so the lights work on the house battery only and not install a switch. No sense in running down your engine battery.
I have a diagram that shows how to wire it so you can jumpstart your engine battery from your house battery with a flip of a switch. Normally, the wiring just allows the house battery to charge off the alternator when driving - and that wiring was usually done with a too small wire.
If you'd like that diagram, PM me with your email and I'll send it to you tonight.
