600+ hp, 9600 miles, pull 18 freeway all day long if i keep it at 75 or less. Break it in, then worry about mileage. Drive it like you stole ot for a few thousand miles and I'll bet it gets better. Better yet, tow something good and heavy.
Totally disregard all mileage issues until at least 10k is on the clock or you know you have been rough enough on it that it should be starting to break in. Most people see mileage increase for the first 25k miles of ownership. Also, you are in about the worst ruck for mileage anyways, but it'll get better.
Driving style has alot to do with mileage too, you know. Be realistic when checking your mileage. If you race from light to light on the floor and complain that your truck only gets 12mpg, well, you know. You have alot of power, and it takes fuel to make power, so the more power you use the more fuel, ect.