No tail light or dome lights 2000 f550

The tail/park lights and dome lights will not come on. The fuses all checked out good. This is a pace arrow vision motor home.
When I connect the trailer light connection to the towed vehicle, then turn parking lights on to the towed vehicle, the motor home tail lights turn on. I have no problems with the head lights/brake lights on the F550, just cannot turn on the tail lights