Bummer dude, but a camper almost as old as I am, I wouldn't know where to find a manual. However, you can find basic 12V electrical schematics and put 2+2 together to apply to your situation.
Get a multimeter and some extra wire, couple alligator clips maybe and start tracing the problem. Lights are either daisy chained from the same switch or from separate switches.
Start at the fuse panel find the light circuit(s). CHeck fuse, check power to the circuit. Go to switch, check power in and out of switch.Go to first light, check same. Find bad leg, check continuity to see which wire.
A manual won't help you out anyway except maybe to identify which fuse is for the lights if that's not marked.
This is basic 12V electrical troubleshooting.
You can do it with some research. And the tech is an idiot. Fishing wires through RV walls is already tricky. Certainly don't want to do more of that than necessary. Plus you'd never know where the problem is, if you havent done the troubleshooting first.
Could be a connection, switch, blown fuse, loose ground, bad light bulb receptacle and not necessarily the wire.
Good luck man!