Have you opened up the electrical box (On a 5th wheel it is on the tongue)and looked at the wiring on your trailer plug.
I am in the process of re-wiring mine, they put 4 white wires (I beleive grounds) together with a large blue wire nut, mine was loose.
Also they use them DAM quick splice connectors, (they should be outlawed) on some of the wires.
It is a great place for corrosion to start and give us problems down the road.
Anyplace where it is possible I am replacing those connecters with true butt connectors.