Mystery "Check Trailer" warning Beep.

[Jimbogood]

I have a 27 ft SunnyBrook 'Brookside' 5th wheel and a 06 Ford 250 diesel with factory brake controller.
I get a random (annoying) "Check Trailer" warning Beep and message on the brake controller. Happens most times on left hand turn, but not always. I can,t cause the problem with any method i can think of; just does it when it feels like it. I have ran down all the usual trouble shooting avenues; wiring shorts , grounding or opens. the harness and couplings all look good. Truck dealer says 99% of these cases are trailer related, but I'm not convienced because there are logic relays involved. Had a pro run through the trailer side and found nada!
Soooo, whats your take? Am I cursed or is there a logical cause to the mystery?

[K-Star]

many trailers have the brake wiring run thru a hole in the frame to get to the wheel. most often the wires chafe from rubbing on the frame. i would check that first. then make sure the pins on the connectors are clean (male and female). They are notorious for corrosion.

[Jimbogood]

Thanks for the reply. I'll take a look.

[falconman]

The Ford controllers are very sensitive to bad connections. I had the same situation and found a loose wire on the truck side plug. I soldered all the wire to plug connections, problem gone.