I see that this is an old thread, but I did a search because of similar problems.
> When I plug the trailer into my pickup, the lighting generally works well - interior and exterior. I rewired the round pin plug to a new modern 7-pin flat spade plug. The trailer lights (running lamps, turn signals, brakes) work that are not blown. The previous owner did something to the reverse lights, because the 4 round lamps in the rear operate as brake lights and turn signals. The interior house lights (hallway, living room, bedroom, bathroom, kitchen, control panel backlights) work that aren't burned out.
> I connected the AC plug into shore power (after unplugging the tow vehicle) and nothing worked. I may have messed up: I put a car battery charger on the battery, which sparked a bit, then blew the fuse at the Univolt (1 - Car Battery [pos] & Circuit No. 5 according to the 1974 Airstream Service Manual).
> I bought a new 12 volt
battery and connected it as well, which also sparked a bit, then blew another fuse at the Univolt (2 - Trailer Battery + [pos]).
> I'm going to buy and try new fuses soon, but I'm concerned: How can I isolate the circuit that is shorted if I'm blowing the main fuses?!?
I have not yet connected the tow vehicle again to see if things are working, which I may try.
Any help is appreciated for this newbie.