If your truck doesn't have LT tires on it, it could be part of your problem. I experienced that with my Tundra. Before getting our Minnie, I towed our old Sunline 28' trailer that had to be in the 7500 lb. territory and the truck handled it OK. After getting the Minnie, I started feeling some instability and after doing some research, concluded that it was the passenger rated tires on the Tundra. Once I put on a set of LT tires and inflated them for towing, my rig was much better and I've had no problems after a couple of years and several high mileage trips.
Take your trailer to the scales and see what it actually weighs. If it is below 7,500 lbs. I think something is causing the problem. If not the tires, then the hitch set-up or something!
OR, perhaps it is the length of the trailer....just looked at your post again. That's a long trailer and a lot more surface area for crosswinds.
