You actually have 3 electrical systems in your trailer.
First the running lights which run off the tow vehicle which would not be a factor.
Second the 120v system which powers 120 outlets, is powered by your city power hookup, and sends power to the house battery and converter which it sounds like you mentioned finding
Third the 12 volt
system which is connected to the house battery which powers interior lights, and fans and is powered both by the tow vehicle and the city power supply.
First, did you loose power to the 120 volt circuits, 12 volt
circuits or both?