I just got back from a trip today with my 2008 Tundra DC with Cougar, and I just happened to have (for the first time) carefully calculated my gas mileage. I went from Boulder Co to Fruita Co, stayed a couple of days, and then back. I kept it at about 65mph on I-70, occasionally going up to 70 when I had to pass a big rig or slow car. I got exactly the same mileage going out as coming back - 16 mpg right on the money. I had nothing on the roof and had only my mountain bike with me in the back seat area + food/water/gear in the camper. It is probably the best gas mileage I ever got doing this trip, and that is because I typically do 70mph and not 65. I learned something about doing 65mph on the freeway, there is no drama. Everyone is passing you and relatively slow speed except the occasional big rig that you have to deal with. No stress.
Also, I love my truck. It does everything from pulling a 10,000lb trailer, to hauling heavy landscaping materials, to camping with my family of 4 and our ATC with a canoe and paddle boards on top and bikes on front and back, to everyday driving without even a whimper. It has never failed me. I call him "Brutus."