You are probably on the right track.
The 12 volt
system powers about everything but the air conditioning, microwave and television.
There should be 12 volt
car batteries somewhere and a 120 volt AC to 12 volt DC converter.
When the trailer is plugged into an AC outlet, the converter should supply power to run all the 12 volt appliances and charge the batteries. When not plugged in, the batteries will supply the power until dead.
Several possibilities to solve your problem.
Locate the fuse panel and make sure all the fuses are good.
Find the batteries and measure voltage with a handheld meter.
Bad batteries can load the converter to the point that it has nothing left to power the appliances. One can test this by disconnecting the batteries, keep the cables from shorting to each other or anything metal in the battery box and see if the lights come on.
If not, you may have a bad converter.
Other than dealing with some unknown modifications by a previous owner, this should get you started.