I just cracked 10k miles. Mileage just keeps getting better now that it's summer, the last few tanks where I'm doing about 20 % city and 80 % highway (I set cruise to 70 mph) I am getting 18-19 mpg on the overhead and hand calculated it around 18 mpg. A lot of times I'll have 500 lb in the bed as well. For fuel, I've been running mostly 91 octane in the summer, 87 or 89 in the winter.
With a truck this tall and wide along with fat tires, speed and air temperature/density make a bigger difference than a standard half ton and their deep air dams and pizza cutter tires. If you are bucking a headwind, going 80+ mph, or it's very cold out, mileage drops in a hurry. The nice thing is you can choose mileage when you want or have that 450 hp for fun and burn some dead dinosaurs.
And just as a side note, the truck hasn't missed a beat in 10k miles. So far so good, but I bought bumper to bumper 120k mile warranty just in case.