[5ER]

Recently when we bought our fifth wheel the battery was nearly dead and we could not hitch without connecting to 120vac. We had no 12v power even when the 120v power was connected. We connected the cable to the truck (with truck running) and still no 12v power. They installed a new converter and still no power. They eventually realized that the dogbone connector to the cable was bad so I got a new converter. But they checked the cable plug connector from the truck and said I had no 12volt power from the truck to trailer. I do have trailer lights and brakes. This may have been this way since I bought the truck and had hitch installed in 2003. Is this possible? When driving what uses 12 volts in the trailer besides the refridgerator? How would I fix this?