Strange problem: I'm towing a Terry 25' 5th wheel. Before departing at oh dark thirty, checked all lights. Drove all day, in the evening re-checking lights and found that I did not have tail/running lights on the trailer. Brake/turn/flashers all work fine. No running lights. Tail lights on the truck are fine. No voltage output at the connector on the truck. Fuses good. I tried to jumper the hot lead from my license plate light into the running light pin, that caused the flashers to stay on steady and the left turn light to go on steady. ?? any ideas. and, yes I did clean the connectors and checked that the wiring was intact.

 

Possible bad ground between the RV and the truck. Suggest running a temporary ground wire between the truck and the RV and see if that changes anything. You say no voltage at the connector on the truck. What is the pin number on the connector that you checked? What are you comparing that to for a voltage reading? A good ground on the truck itself, or another pin?

 

Not sure what year your truck is, but many that are equipped with trailer tow will have a complete separate tail lamp feed to the trailer, powered by its own relay and/or fuse.