With 120V AC power to converter (via a circuit breaker or plugged into an outlet) the converter DC Output should be 13.2V DC Minimum.......otherwise converter is/has failed.
Truck Alt will supply a 'charge' to camper battery via the charge line/pin in a 6 pin or 7 pin connector provided the fuses/relay in truck fuse panel are in place and line connected and truck is running.
6 pin or 7 pin charge goes to camper battery NOT thru converter.
If you have a battery disconnect switch it has to be in proper position so that camper battery can be used/charged whether from truck or from converter.
Plugged into a 20A outlet is OK
Trailer wiringConverter Replacement----ASK Randy