Hi Bill,
I am not fluent on the old Sunlines, but I am on the newer ones. See if this helps any.
On the newer Sunlines, all the DOT wiring runs to the back of the camper under the camper (on the outside of the bottom cover) from the 7 wire truck plug connection junction box to the back wall area. Then the bundle goes up "inside" the back wall and fans out to the top red clearance lights, the left and right tail light and the rear side clearance lights. And on the newer campers, there is a frame ground at that back wall bundle where it goes up into the wall.
Do not know if yours is anywhere close to this. But would say this.
Start at the 7 wire truck plug. Trace that back into the camper where is goes into a junction box. The older ones had a wooden covered junction box with a cover on it many times up front inside the camper. Look under a couch, cabinet etc for a wood cover. There is often 2 fuses in there too. One for the truck battery charge line and the other for the camper battery power line to prevent wire burn outs/shorts etc.
If you can find that, then you found the start of the wire bundle and the truck ground wire. The truck gnd wire will go back out to the frame ground up front. That can be all corroded up. Then look to see how the tail lights, rear clearance lights etc. head out of that box and work their way to the back wall. If you think you lost ground, pull a long temporary wire starting at the 7 wire cable truck plug junction box, out of the camper and back to the fixtures that are not working. Figure out which wire in the fixture is hot and gnd and test it there. You can also use that wire as a continuity test to right out the tail, clearance and stop lights using an ohm meter or test light as they all junction in that 7 wire truck plug box. And being that old, someone may have modified what Sunline provided as well. But they all need the 7 wire truck plug if the camper is big enough to have electric brakes.
This is the color codes Sunline used on the newer campers. Look at the center one called "7 way RV standard" wiring. Yes the RV world uses different color codes then the general trailer industry. But they all work in the same 7 wire truck plug. Odds exist they used the same color codes on the older campers.
Hope this helps and let us know how it goes.