Had a similar issue with my 22 F350 Lariat, when I picked up a new 5th wheel. Lights would not work, except right rear brake light. Dealer was adamant it was in the truck. So off to the Ford dealer I went. Talked to a tech from the back, he was adamant it was in the trailer. Headed home. Call Grand Design. They said to go over all the wiring in the trailer. I did find a wire that was not seated all the way in the lighting module on the trailer. Made the repair, and it has worked ever since. The trucks are sensitive to any wiring feedback it doesn't like. With these Ford's, it's all about the truck.
Also, when driving down the road, sometimes I will hit a bump, and the "Trailer Disconnected" warning will come on. But it seems to clear itself.
So, you're not alone.