I pull 13 mpg almost religiously. Now, keep in mind that I have 37" mud tires on a 4" lift and 4.56 gears. So, I am going to get 13 whether I am in town or on the highway. PRobably get that pulling a decent trailer too. When I was completely stock, I was pulling 16/17 on average with some of my mileage hitting 20.
Also keep in mind that I have done a few mods to help the mileage out:
-e-fan conversion
-ported/polished intake
-modified air box with K&N filter
-true dual exhaust with a Flowmaster 70 series muffler
Another thing to keep in mind is to keep your RPMs between 1800-2200 RPM as you are cruising down the road. To go above that puts the engine out of its efficiency band and going below pulls it out of the power band.
When accelerating, depress the gas pedal such that by holding your foot in 1 spot, the truck will raise the RPM until it hits about 2500 RPM and then shift. While you will not get lightning fast acceleration, it will be tolerable and save you a surprising amount of gas. The other big hint, when you see a traffic light change to red in front of you, immediately let off the gas and start coasting. Saves a fairly significant amount of gas and is also a lot easier on your brakes.