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Hi my name is Robert...
What I would do is get a test light and then probe the wires on the trailer side and see if you have power coming out of the plug on the trailer side, could be a bad connector or plug on the trailer side, follow the harness going out of the plug on the trailer side and see if you have a tow plug fuse card, sometimes on 5th wheels they will have there own fuse card for the lights so try and follow the harness and check any fuses you see that are in-line with that circuit. Go to the battery and also look for any blown fuses as well. IF you still need more help please let me know, you can follow up as much as needed.... Robert