I'd try and find that 4 wire flat connection at the end of the tongue harness, its either that connection or the mice likely exposed some wiring in the tongue and its shorting out the harness to the tongue.
None of the light fixtures can get ground from the trailer metal itself, its all from the wiring harness, pull any of the lights and there will be a white ground going directly to it. Triton does not connect any grounds to the trailer metal, so trying to ground the frame isn't going to help anything. Its all solid connections to every fixture from the 7 way connection at the truck.