If you couldn't get the wiring diagram from the guy that built the camper, and you can't provide a brand and model to us, you're sunk.
Really it's not that complicated. Treat the lights like one wiring system, and the 12V interior electrical like a completely separate system. The only place they will connect together is at the 7-pin RV plug that connects to the truck. Even then, the only COMMON connection will be the ground (battery -).
For the internal wiring, think in terms of a wiring "bus." All the +'s get connected together (12V+ from truck's trailer plug, converter 12V+ output, house battery +, lighting, appliances). All the -'s get connected together (ground from truck's trailer plug, converter ground, house battery -, lighting, appliances).
You'll want to separate the + connections for various appliances and lights onto separate circuits with a fuse panel. AutoZone has a nice 6-circuit fuse panel that uses automotive type fuses.
All the - connections can be made together on a bus bar.
Putting 10-ply tires on half ton trucks since aught-four.