Airstream, the fiberglass eggs (Casita, Scamp, and the like) could yield better gas mileage. They're nicely rounded at both front and back and often also have a lower profile.
All of the others, you're still towing a big wall down the road. An aerodynmic-appearing front doesn't help much if the rear is flat - nothing sucks like a vacuum. (Point of reference - I get 25-26mpg not towing, 11 mpg towing. This is about the same mileage that much less fuel-efficient vehicles get when towing the same trailer.)
The most effective other way to save fuel: slow down.
