PerryB

Jeez, now you sound like ME !
I hear you. I think I've rewired every commercial built trailer on the ranch due to lazy/sloppy manufacturing. On a couple I've actually run emt and pulled the wires through that. I also like to put a junction box somewhere in the tongue area so when (not if) the plug end gets torn up it's an easier fix.
Your idea of load testing at the fuse wins the prize. I seriously doubt there is a ghost/parasite load on that circuit, but that would be the simplest elimination.

PerryB

Delete /duplicate

F175

Try another trailer.

Flamingtaco

I use the plastic breakout boxes when there is a triangle, and water from there, with junction boxes at every point wires need to break off. I run the conduit into the frame tubes and secure it so it can't fall out. Dump trailers I skip the breakout box, run the wire inside the tongue bar (inside conduit), and terminate at a junction box. I haven't owned and dump trailers with brakes, just smaller ones with clearance and brake lights. If I get one with brakes, I'll probably install a breakout box for troubleshooting ease.

I've never had to troubleshoot a trailer I've rewired. For an extra $10-20, almost every trailer wiring issue ever could be eliminated. Nah, build 'em cheap and fast.