Saab, your trailer plug lights are separate from your MH lights. The trailer lights are all controlled through your Bussman box which is a taillight convertor also (from sep brake & turn to combined). They are a black box, about 4" square and there are two of them, one for trailer lights and one for the trans. Take off the Bussman box cover and you will see the fuses and relays for the brake and turn signals, etc. for the trailer plug.
They are usually mounted on one of two spots, under the bed above the engine mounted on the cross bar, or, in your electrical bin on the wall. I can post a pic if you have a hard time finding. Welcome to the forum also.
edit to add: Bussman is a mfr so this name will be on the outside of the box.