I would wait until the truck has 2 or 3 thousand miles on it before making a firm judgment about the mileage. A lot of folks on here are saying their gas mileage improved after a few thousand miles.
I agree with BeauxXL1200, give it a few miles. I just turned 4000 and seems to be a little better, I only see under 14mpg first thing after startup when the ambient temp is cool. Once engine/trans warm up the MPG gets better. Computer shows me getting 17-18 a lot of the time. On a good day it goes up from there if I drive it right.
Attached pick of headed home from work a week or so ago, this is about as good as I ever see on the interstate without a tailwind, notice it was just about to turn 4000 miles in the pic.
FYI I only use 89 octane, I initially started on 87 and either more miles, more octane, or some combination made a difference. And I live in a very flat area which helps a lot.