MPG is inversely proportional to work being done. So, when I hook up the fifth wheel I'm moving 2.5 times as much weight. However my experience seems to indicate that aerodynamics of the trailer is big factor at the highway speed that most of us travel. Our 2012 F-350 CCSB SRW 6.7 diesel gets 20 at highway unloaded, 15 around town, and 11 towing at highway (+ or - depending on wind direction.) I've read many reports that the 20 -21 Fords are getting better mileage with the 10 speed and lower numerical axle ratios. But when towing I'd be surprised if they do much better due to the significant amount of work being done. As an indication of the effect of aerodynamics at highway speed our 1) 6500 lb 27' bumper pull trailer, 2) 27' 9400 lb fifth wheel, and 3) 37' 12,700 lb fifth wheel all got around 11 mpg at highway speed. The fifth wheels have/had round nose caps while the bumper pull was classic old flat.