I had 2 shorts:
The original old-style charger between the water tanks had been cut out, but its + wire was still on the positive post and showed continuity to ground.
The wiring harness from the 7-way connector splices the alternator charge circuit into heavy cable that runs back to the trailer battery. The splices were crimp with heat-shrink insulation, which was good, but they were improperly crimped with crimper tool jaws for non-insulated connectors. Those pierced the insulation, and eventually with moisture and electrolysis corroded into a fine shorted mess. These splices are zip tied to the tongue if you want to check them. Mine were fulla’ green gick!
Once I realized what these issues were they were easy to fix with new waterproof splices correctly crimped and shrunk, and removed one wire from the positive post.
This trailer’s previous owner hit many salted roads and wasn’t able to keep the battery maintained over winter, kind of the extreme of rough conditions. I’d guess road salt contributed to and exposed both problems.