I agree that wind resistance is the major factor towing a full size TT. It's an 8 foot by 8 foot parachute, and no matter what you drag it with, you are going to get between 8 and 15 depending on how fast you go and whether you are gas or diesel. Also, whether you have a headwind or a tail wind.
I've had two diesels, and the worst I've gotten is 9, but that was towing into a 50 mph headwind (I'll never do that again!) and the best was 16, with a tail wind and going slow. And yes, both of them get around twice the mileage when not towing.