Depending on the year there is either a fuse panel on the univolt itself or a remote fuse panel on the trailer. I would check tose fuses to see is one is bad. Most trailers had 2 or 3 individual circuits for lights, fans, sockets, etc.
If your unitvolt is working then it should charge the 12 volt
battery and create an additioanl AMPs to run the other 12 volt
stuff that is on.
The old univolts had a set of screw in glass fuses that as they heated and cooled would cause the univlot to disconnect from the 12 volt
system. I would check for this too.