When the dealer did "all they could do", were they testing with your P/U, or theirs? If theirs, then I would tend to agree that there may be something going on with your truck, or minimally, the wiring connection between your truck, and the trailer. Is the brake controller a built-in, or add-on? Maybe not getting good enough connection between your truck round plug, and the trailer connection. Is the gain turned up high enough? Could it be the trailer pushing the truck?
Do you have a second vehicle that you do a test tow with? Or does a friend have one? You need to narrow the problem down by eliminating either the truck or the trailer as the problem.
What happens when you use the hand controls to apply just the trailer brakes? A technique I use occasionally when I adjust my own brakes, is to take it for a test drive, and intentionally "heat the brakes up" by using the hand controls, and then stop to see if they are all about the same warmth, indicating that they are all adjusted about the same. You could use that same technique to make sure all wheels are contributing to the stopping process. just a thought.
I don't know, just questions.
