I was also going to suggest that you use a volt/ohm meter and verify that your system is grounded. On most RV's the electrical
system is grounded to the steel frame, and the vehicle system is grounded through it's steel frame or chasis. For the lights
to work there has to be an electrical
ground between the trailer frame and the vehicle chasis. The lack of a proper ground is very often the source of electrical problems in an RV. One wire in the electrical harness connectors will serve as the grounding wire between the two units.
As mentioned above, this is usually the white wire, but not everybody plays by the same rules, so if you can get an electrical circuit drawing for your trailer it will identify the circuits for you, by colour coding.