 I bought a 2007 F350 Super Duty XL 6.0 L Diesel, Turbo, 4Dr, 8' Bed last summer.  I needed a heavy duty truck for carrying my 3420 lb. cabover camper and pulling my 3500 lb. 2-horse trailer.  I overheat on every hill just going to Payson, Az.  Had to pull over 3 times with my horse behind me!!!  If it were just myself I would have continued the trip to Pinetop/Lakeside. However, I had no choice but to return home, (going downhill) because of the safety issue for my beloved horse!  Already check the coolant.  Will try a new thermostat but am afraid it may be a head gasket.  I live to camp in the summer and here I set because I already spent my money on the truck!