Why not follow the previous advice and tow the trailer with a friend's truck? If that cures the problem it is your truck that is the culprit. If not then the problem is with the trailer.
Or, tow another trailer with your truck and see how that trailer handles. Before you throw any more money at the truck, it is my suspicion that it has nothing to do with the truck.
Sometimes airbags complicate the WDH spring bar loading. Try it without any air and see if that helps. But first, like so many others have stated, get 13 percent or so on the tongue measured correctly by a tongue scale.
If it starts to sway, use the manual override on the controller to activate the trailer only brakes. That should get the trailer back in line.
