A major factor in fuel economy when towing a travel trailer is the frontal area of the trailer. It takes energy to push all of that air out of the way and that's more of a factor than weight.
I have a 24' enclosed car trailer and I get the same mileage empty as I do with a 4500# vehicle in the trailer. I also drive charter buses and we pretty much average about 6 mpg whether we're loaded or empty. I think if you really want good fuel economy, get a pop-up or a crank-up that doesn't have the frontal area.
Bill J., Lexington, KY