should work when plugged in to 120, but better to have battery installed. Also 12 volt
should work when tow vehicle plugged in. Maybe a silly question, but does your trailer have a 50 amp glass fuse in the battery neg. wire, mine did, it was mounted on the wall just inside the access door to the left. Mine had a dirty loose connector and until I found it I had intermittent 12V problems. Good luck.......Phil.