I think you just need to get used to it being a truck. Trucks really aren't built for gas mileage... never have been.
I have a 2006 K2500HD with a 6.0L and if I'm lucky I get around 9 to 10 MPG. When I'm towing my 5th wheel... about 6 to 8 MPG.
Only modification is a Dynomax 4" exhaust system, which I hate... It is way too loud.
Gary