As another engineer I have watched this thread. I don't have enough information to make a single suggestion. I do have an observation or so. Jayco used an oversized TV for a test pull of your unit, but you may have missed the logic behind this choice. In testing you attempt to limit variables so as to focus your test as much as possible. There are many variables in towing, it's a system made up of variables, the TV, the hitch, the setup of each and a trailer. Also, a single axle trailer is a simple machine in that if the axle is not bent and you pull it from the center it follows the pulling forces. It's like a hand truck, you grab it in the center of the handle and walk and it follows you. The actual shape of the frame would make no difference as long as the simple things I mentioned are there and within spec. You could build a frame of almost any random shape and given a straight axle, two good wheels and the pulling forces close to center and it will follow you down the road. So given this, is it possible that when they told you they have no idea what's wrong, they actually have no idea? Is it possible that issues with the trailer have been eliminated as the cause of your problems? I am just asking because I have been there, I have made this same type mistake of logic many times. I do hope you find a good resolution to all this.
No man has a good enough memory to be a successful liar. Abraham Lincoln
2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch