In 2011 we took a popup TC to Alaska. My old truck Chev K2500, 7.4, w/3.73. On a good trip I'd avg 14 hwy empty, normal running around 12 mpg. For the Alaska trip weighting in at 11,000 gross I avg 12.43 for 13,000 miles.
This was all kinds of of driving. The Al-Can hwy, Top of the World hwy (90 miles of mud road), The haul road to the Arctic Circle. Down to San Fran, Reno, the Bonneville Salt Flats, up Pike's Peak, Down to NM, TX and back home to IN. Dessert, Mountains, hills and flats about as diverse as it gets. Truck had 19.5" tires, 100 gal aux fuel tank, Air-Lift bags and the 10' SunLite camper. I did make air deflectors from the cab to the camper as it was set back a foot by the fuel tank. I also filled in the space above the truck cab with swim noodles in a V shape.
The Popup TC was the perfect camper for our trip.