I used to get 10-11 mpg with our Passport 285RL (around 5000 lb.) towing with a Dodge Ram 1500 HEMI at 65 mph on the interstate. Of course, this was in GA and FL; no steep mountains, except in N. GA (and they're not that steep). Wind can definitely affect gas mileage. We always get slightly better mpg heading south down to FL than on the return trip. Even though it's not mountainous, there is a slight grade heading north from sea level up to the beginnings of the Appalachian Mountains. There always seems to be either a head wind or a crosswind, too on the trip back north.