Looks like possible pinched wiring in tail/running lights...

I would start first by inspecting every clearance and tail light fixture. It's more likely that one of them is broken or has a pinched wire than the wiring harness itself. It's also easier to do than following wire under the frame and/or inside walls.

That said, anything is possible. My sister's trailer had the main heavy-gauge battery wire pinched between the 2x4 baseplate of the wall and the top of the frame. It was that way for years until it finally wore through the jacketing.