Fist step - check the ground.
Make sure you have power from the running light circuit on your truck socket. Connect your test light clip (see tools) to a good ground and probe the contact on your truck socket at the 10 o'clock position. (viewed looking into the socket toward the front of the truck).
If you don't have power there, look for a blown fuse or a bad connection somewhere in the truck. If your truck running lights work, then the problem is either at the socket connection or the wiring between there and the main chassis harness.
If the truck socket shows power, then the problem is in the trailer wiring.
If some of the running lights are working, continue below.
If none of the lights are working and the fuse blows go here.
Here's a "quick and dirty" way to check the trailer running light circuit: Your trailer must have a charged battery installed. Buy an in-line fuse that has two wires leading from it and insert the wires into the top two contacts of the trailer plug. This directs the current from the trailer battery to the running light circuit and the lights should come on. The fuse will blow if the circuit is shorted somewhere. By the way, this is also a handy way to light your trailer if you have to leave it at the side of the road, at night, in an emergency. To have your lights flash on and of, in this situation, you can make an inexpensive emergency flasher.
If you have one or more running lights operating, at this time, then your trailer plug is probably alright. Check each nonoperating light, in turn, for a burnt out bulb, corrosion on the bulb and terminals or a bad ground connection. I've found a good tool for sanding the bulb or terminal surfaces is an emory board (nail-file board).
Older coaches with metal siding often relied on the trailer "skin" to provide a return ground path for the electrical current. This ground connection was provided through one of the light mounting screws. To see if your coach is wired in this way, remove one of the running light assemblies and note the color of the wires behind it. If you have one or more green wires and no white wires connected to the light, you most likely have this type of lighting circuitry.
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