TCs "suck" the fuel. I'm on a month long trip right now and have had a great tailwind from Lake Havasu to Amarillo tonight. could tell by the feel, sound and boost gauge that I wasn't using much power, but I still only got 13.5 mpg over 400 mi. I was towing a 4k lb pontoon boat but a TC like a high profile Lance just has a lot of wind resistance.
Speed is a killer. Double the speed, quadruple the drag.
I'm just glad I was going east not west. Although, I do have to go home.
I nomally get 10-11 mpg with my Lance 981 and 03 Dodge CTD.
It's funny how towing a 12k lb 5th wheel gives about the same mileage.