Have you made sure that all the wires that should be connected to your trailer plug socket are actually connected? Are your tail and marker lights all functioning properly?
Another possibility is that it might have something to do with a PO's toad braking system. My ReadyBrake toad brake system uses a pair of wires, one hot and one ground to activate an LED indicator light on my dash whenever the ReadyBrake is applied.
Have you checked the wiring diagram for your MH?
Wiring Diagrams