I towed my TT with a previous F-150 that was 0-500 pounds over GVWR depending on the trip. If you return the front axle to unhitched weight, the rear will settle about 1.5-2" lower than unhitched height. Any more than that, and you exceed GVWR and probably rear axle rating. There is a reason why the rear springs are soft, the vehicle isn't designed to handle more weight!