Let me enter the discussion one more time. Here is a possible path to solution.

Make up a jumper wire (single) with sharp pointy thing at each end (awls with alligator clamps to each), length determined by the trailer. With directionals, or emergency flashers (my preference) on, start at the hitch, and using the sharp point, make contact thru rust and paint with the chassis of the trailer, and put the other end on the tow vehicle. You are looking for the lights to stabilize...only the directional/flashers to work properly, and the other lights to remain with a steady glow.

Follow up at each light, stop/directionals and marker. If the main ground tested OK, each of the lights can be checked at its ground site. When you reach the bad ground area (assuming it is only one), the problem will go away.

If you are lucky when you do this: Correct the problem. Leave the rest of the wiring until later.

Good luck.

Bill