Joe,
I even get better than that with a 35' RV weighing 19,500 lbs and pulling a double stack. I got around 9.5 mph on the trip up to the F18 Na's driving around 70mph. On the flat lands I can get 10 to 12 mph and better if going slower. My F250 gets around 15 to 17 towing at 70 mph.