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Turn just the parking lights on and look at the front of the motorhome. The lights to the outside of the headlights should be on. Let me know if any of the marker lights or parking lights are on. Walk all around the motorhome.
Just the parking lights.
The parking lights and the running lights are all on the same circuit.
For the trailer lights, check fuse "P" in the engine bay fuse box.
But since the front parking/running lights work, you have a broken wire for the parking light circuit between the headlight switch and the rear tail lights. The ground to the tails lights is good because the brake and turn signal lights work. There should be a connector under the rear of the vehicle on the frame. That is where the aftermarket motor home companies tap into the factory harness.
The parking/running light circuit is the brown wire.
The connector should have 12 wires in it.
Do you have a meter to do a few electrical tests?
The wire could be broken between the connectors in the harness running along the frame or up in the body of the motor home. There is no common place for the wires to brake. Since it is an RV, the wires are running different than factory Ford harnesses are once they enter the body.
Sorry I could not help you locate the problem. But to properly find it, you will need a meter to check for voltage through the wires from the tail lights back to the headlight switch. There is only one fuse for the parking/running lights and since the front parking/running lights work, the fuse is good and so is the switch. That only leaves the wiring.