I just got back from a 4200km trip, most of it towing. We wound up in the Watkin's Glen area for the Nascar race, Lake George after that for a week and then Normandy Farms in Foxboro MA for another 6 days - through all of this towing my truck averaged 11.1 MPG indicated and I rough calculated every fuel fill-up and was about in the same range.
Lots of grades during our travels through Vermont and upstate NY as well as through northern Maine on the way home.
Truck worked fantastic and was a pleasure to tow with through much rain, hills, wind and rough roads.