If you bought a 7,000 lb. truck for a daily driver with lots of short trips then forget about your mileage and enjoy the truck.
I get 19 MPG with my 2011 2500HD 4WD truck empty and 14 MPG with a 4,000 lb. load in the bed. My daily driver is a Prius sedan that gets 45 MPG on regular gas which costs 20% less than diesel.
I would not put 4000 lbs. in the Prius but I also do not expect great mileage from the heavy duty truck. If I put $2k into modifying the truck I void the warranty and it would take 100k miles to recover the $2k with a 15% improvement in mileage.
With the older trucks tuners could improve MPG but with the LML I get better mileage than these older trucks and more power so why bother?