A 91 should have a converter. I would check to see if you have a blown fuse. If you do not have a battery make sure your positive battery terminal is not touching the trailer, otherwise it will just pop the fuse again. Like another poster said you need a battery for the breakaway switch when towing. Your lights, fan, heater all run off 12 volts.
If you have a VOM or test light you should have power to the battery terminals when plugged in, (Without battery).
If you put in a battery you need to disconnect when not using unless plugged in ( converter should keep it charged) otherwise it will be dead in about a week.
You need to make sure the converter is charging the battery.