Very slim chance of finding a factory wiring diagram, most RVs of that era were slapped together so fast they had really no idea where things ran.
Do any of the interior lights work? It's possible that the wiring came loose at some point. If you remove the light fixture can you follow the wires? Pull gently to see if they might be disconnected. Have you tested each light fixture to see if there is power? If possible trace to see which fixture is the closest to the fuse panel.
If you haven't swap the fuse to that section.
Above the ceiling is usually enough room to use a hand held mirror to see along the roof, might see if you can follow any of the wiring.