It's hard to say what kind of mileage you will get. A lot depends on truck configuration (crewcab/supercab/regular cab, longbed/shortbed, and especially 4x4 or 4x2). My parents tow a 12K fifth wheel with their F350 dually 2wd and get about 14 MPG on the highway.

Another key to mileage is how you drive it. If you're up above 2000 RPM quite a bit and punch it when you accelerate your mileage will be significantly lower.