I have not heard anyone with the new powerstrokes getting much over 15 unloaded, they are thirsty. One lumber yard up here has a f450 with maybe a 12' flatbed, dually 4x4, and they can push 10mpg with it if they baby it real easy.
My last work truck was an 08 Dodge 2500 crew cab, long bed, 4x4 with the 6.7L, in 1 year we put 110k miles on it and I bet you at least $15k worth of warranty work( but that is another story). But that truck is dead nuts stock right down to the tires and it will easily see 19mpg unloaded on the highway. After about 50k miles the mpg's started to get noticably better from when we bought the truck. Its a lease truck so we can't modify it in any way, which sucks.
My last person diesel truck was a 2000 F250, 7.3L, auto, 3.55 gears, 4" lift, 35" tires, with an exhaust, with 55k miles on it. I could honestly count on between 17-19mpg on the highway, and a solid 14mpg pulling my jeep on a flatbed, about 6500lbs.