Hi Drifter,
Welcome to Sunline Owners Club. Glad you found us.
WOW that is a rare find! Congrats!!!
Just need to be in the right place at the right time...
On the front clearance lights, since you did the hot lead trick that helps eliminate the grounding at least. I do not know exactly how your camper was built but I will go on the assumption Sunline used the same practice as with their other campers.
So here goes, find the front most body/clearance light that works correctly even if it is on the front side of the camper. Remove the screws that hold the fixture to the camper. There may be a gasket or putty tape sealing it to the camper, pry gently. What you should find is both a ground and hot wire stuffed in the wall, wire nutted to a daisy chain. While the lights are all in parallel, they jump from fixture to fixture with both the hot and ground.
Undo the wire nuts on the hot wire, if you have an ohm meter you can figure out which is the ground by the wire being common to the camper ground. Or use a test light to find the hot one. You will need to figure out which lead is the incoming hot wire and which is the leaving hot wire from that location. With the hot wires loose, power up the circuit. If you found the actual last location as the fronts go completely out, that location then jumps up to the top front clearance lights, then good you know where the wiring starts.
What you are not going to know is which side front clearance light is the last one in the circuit. If there are only 1 hot and 1 ground at the first fixture you pull, then it "might" be the last in the loop, go to the other side and try.
When you find the last good light that you know then jumps up to the top, check for corrosion at the wire nut and if it is is good then go up top what looks like the first light they would of jumped too.
It will be a process of elimination until you find the last good hot wire location to where it jumps to the top front lights. When you find that wire hopefully it is just corrosion or loose at the connections at either end of the hot lead wire.
If you for sure have traced it to that section of wiring being bad, somehow in the wall it got rubbed etc. Odds are low but not impossible it rubbed through in the wall. If you are real fortunate, you have the 2 ends open and tie a fresh new wire on one end, solder it on etc. so it will not pull off and hopefully you can fish a fresh new one through when you pull out the old. Sometimes they tie the wires in a loop/not and then your just plain stuck.
The "hope" is, it is just a wire nut connection that is loose or corroded which is a quick fix once you find it.
I would not tear into the wall as only a ultimate last resort and even then I would think of a better way. If I had to, I would go in from the outside siding as it will comes off and will not totally destroy itself. The inside panels are glued to the studs, the siding is stapled and you can undo the staples. But lets leave this as a very distant foggy thought right now.
Hope this helps and good luck
John