For months I have been fighting this message every time I hook up the gooseneck.  The brakes worked, the lights worked, and still I got that annoying beep and message on the dash.  I tried cleaning the plug on the trailer, cleaning the receptacle, checking the breakaway switch, looking for loose wires, bad grounds, really anything that might be the trouble.  Last week I realized that there is a plug that I never checked; where the wiring harness connects to the back of the receptacle on the truck.  It was corroded.  A few seconds with contact cleaner spray, let it dry and put it back together, and problem solved.  So if anyone else is chasing that problem, check out the back of the receptacle on our truck.  Maybe it can save someone some aggravation.