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I have towed a 31' Keystone Cougar with dual slides. Empty weight around 7,100. Probably loaded in the 8,500-9,000 lb range. I have towed it with a 2005 Expedition, 2009 Expedition and now a 2016 Expedition. With the exception of the squat on 2005 and 2009, the Expeditions have handled the weight with no problem. However, squat was always an issue. I researched the heck out of it and found that there is really no solution. Maybe stiffer springs, but that brings other issues. Because of the coil-over-strut design, no aftermarket air suspension is available. My 2016 I ordered with the new self-leveling Nivomat style shocks. They work perfectly! Hitch up, it squats. After a few miles driving, the shocks level the truck perfectly. The Nivomat design is completely stand alone-no wiring, pumps, electronics. Just the shock. This is a long shot, but I wondered (when I had the 2009 Expy) if the shocks could be purchased and installed on an earlier model Expedition. I'm not sure who could answer that question, but it is the simplest solution that I never attempted.
How many RPM were you up to when going uphill? MPG? That's a mighty big trailer.