When we took a long trip a couple of years ago, 3,800 miles, we got 12 mpg for the entire trip. Our FW is 30' long and fairly light, about 7,200 lb dry and up to 9,950 fully loaded. We probably pulled around 8,000-8,500 pounds on our trip. When we also hook up our 19' crestliner behind the FW and go to the lake up to Grand Teton National Park, we are towing probably 12,000 to 13,000 lbs and we still get right at 12 mpg. We tow with a 2012 GMC D/A crew cab short bed pickup. Very happy with the mileage. We have a little lower front profile and I would bet that helps with our mileage compared to some of the bigger, heavier FWs out there.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2