My mileage towing our 25' Surveyor has gotten a little better since I put the new Leer shell on (from Tom's Camping World there in Surprise, Az). I used to get 10-12 consistently towing at 60 (my comfort speed) and now I'm getting 12-14 (ok, so not going up hills!). I no longer notice any sway or movement when semi's pass me going either direction. Great investment. (We spend a lot of time around Williams and Flagstaff camping and going up those grades from the Prescott area to get there I usually get 10-12mpg depending on weather, summer or winter gas, etc.)
Sounds like you have a nice camper too! Enjoy!
2013 Forest River Surveyor SP220 Sport
2012 Ram 1500 4X4, 5.7L Hemi quad cab