I'll agree with the others that how you drive and wind resistance are both much more important than weight. I had occasion to drag a car on a dolly 200 miles and back a couple years ago. Weighed almost as much as my TT. I got 18 mpg towing the car/dolly, and I get 14 towing the full height/width TT. Beyond that, I towed that TT into a 50 mph head wind on two separate occasions, and got 9 mpg once and 10 mpg the other time.
If you are wanting to improve mileage badly enough to replace your TT, I'd suggest you look into something like a Trail Manor that has a lower profile while towing before you consider a lighter rig.