We have an '06 5.9 dually (3.73gears) that we use to pull our 38' Montana, my total gross weight is around 21,600 give or take a tad. :W
I can average around 10, 10 and a halfMPG pretty easily, sometimes better sometimes worse, all depends on the hills and the wind direction. The best I ever did with the trailer on was just under 13 MPG, must've been a heck of a tail wind!
Watch the prices on the 2011's what Chris said is good advice, maybe hold off and spring for the 2014 "heavy duty" beefed up frame and DEF when you have the dough.
Dan