If you are looking at instant readout fuel consumption, you are getting just that. A snapshpt of what the vehicle is doing at that instant. If you were to take your foot off the gas and coast you would suddenly be getting 30 or 40 mpg. It's not an accurate reading of your towing mileage over the course of a trip. Hell, my Suburban, 496, 4x4, 4.10 gear, gets 8-9 towing a Top Gun.