Not a clue, but here are some stray thoughts:
I thought the ground for the clearance lights was with the rest of the 7-pin collection, not the trailer's frame. ??? With a long time since use, I'd suspect the 7-pin connection between trailer and truck where sometimes you can wiggle them and now they work. Other times it is the ground on the truck that has failed.
The trailer frame is ground for the 12v side and the 120v side.
The GFCI problem might be from washing the trailer now that you want to use it to go camping and water got in the outside receptacle.