So, when you say "the voltage dropped to 8 volts," do you mean that before the short to the cover, you could put a VOM across the terminals of the battery and it read 12+ volts, but now, even with everything disconnected from the battery, it only reads 8? If so, search no further--your battery is wrecked, go buy a new one.
As far as fuses go, you may have to remove a little panel on the end of the Univolt to expose the DC system fuses. That being said, I don't expect that any fuses got popped when you shorted the battery. You created a very short circuit between the two terminals, if anything, the other (legitimate) circuits in your trailer should have been seeing less potential than normal (unless they were popped in the first place).
Now, the issue with the tail lights should be unrelated to your battery woes. In the umbilical that controls your signal lights and brakes, there is also a charge line that runs back to the battery. This charge line allows 12V
from the tow vehicle to go back to keep the house battery topped off while towing. So when you plug into 110V AC, your converter (the Univolt) is getting the 110V and converting it to 12V
, and delivering it to the same circuit that that 12V
charge line is on coming from the tow vehicle. Somewhere in you wiring, the charge line is touching the tail light wires. Be aware that the wiring pattern in the vintage trailers like yours does not match the modern standard, ie., the pin at position x in your umbilical does not operate the same thing that it does today. You should start out by taking a good look at the plug/umbilical and figuring out which wires go to what (and especially which one is the charge line, and what else it is touching), and making any needed modifications to make the trailer compatible with your tow vehicle wiring.