A heavier truck, lil bigger than 1500, lifted, 35's, pushing more mass, you are not gonna get Prius mileage, just saying. Yours may be a bit low, but driving habits, conditions, speed, temps, traffic, your particular habits on driving, even PSI and good tier fuel all average in there on MPG, just a saying.
ADDED: Now I just got a 1500, stock height, Stock tire size, but heavier AT3W tires. I try to not destroy the skinny pedal, no hard take offs from lights, drive reasonable, carry no extra weight in truck, use good fuel, keep an eyeball on psi of tires (when cold), almost always use CC on highways, and I can get 13-17 in town, and maybe low 20's on highway, but usually 18-19 ish, and a constant 12.2 when towing total of 6-7,000 lbs.. But a lot goes into driving habits and conditions, etc.
ALL above IMHO .....
ADDED MORE: Unless the MDS system was tuned out, you may not even notice it, especially with the 6.4 more than a 5.7. Run in manual mode gear 8 and that keeps the MDS off, but U have to go into it every time U put truck in drive. Again, back to conditions, truck conditions and driving habits.