I had occasion recently to integrate our horse trailer to my new STX. It involved wiring one of those round 7-pin connectors to the trailer harness. Everything including the trailers electric brakes worked fine and the truck declared that there was a "Trailer Connected" for the test drive. Trouble is, it later declared "Trailer Disconnected" on several occasions thereafter. Each time I checked the rig's integrity and found it all OK.
It occurred to me that perhaps the ground return path through the hitch and ball (and receiver) may be intermittent. I had painted all but the ball of the receiver and the trailer (female) hitch component was a bit rusty. My guess is the truck computer continuously checks the integrity of the ground path as it goes into the trailer connector to it's corresponding structural presence carried from the trailer frame through the ball back to the truck. Unless this is continuous, the truck monitor declares a disconnect. Am I right about this?

 

All trailer harnesses include an integrated ground. In the case of the 7 pin, it's generally a white #12 wire. Regardless of the grounding of the ball/hitch, you should always have a reliable ground. I don't know why you're getting a "Trailer Disconnected" message, but I would count out the ground as a possibility.

 

Maybe try a different trailer first and double...triple check the wiring. Ask a neighbor or relative to try this. check for corrosion at the plug too.

 

I have had this problem and it was a ground with the trailer ball issue -for me at least