what kind of driving? how is the truck setup? (gears, engine, additional weight?) How about your driving habits?
Decreased speed and less stop and go will definitely increase mileage a measurable amount.
Even in my girlfriends car, that is capable of at best 32MPG@60MPH, she typically got 26MPG combined city/highway. I can get on average 30MPG out of the car in the same driving (same route, same time of day) the only (major)variable being the driver.
I can't stress enough that the way you drive is the largest influence on your MPG. Racing to the next stoplight and using your brakes, or racing to the back end of the car in front of you, is extremely wasteful of gas, and is amplified in heavy vehicles like we drive.
Since aerodynamic is a word GM engineers didn't know when these trucks were designed, (actually still true of trucks) the higher the speed, the worse your mileage will be on the freeway. True of cars and trucks, just that in trucks it is more pronounced.
With nothing other than a well tuned carb and a mild 350, my K5 got a best of 18MPG on the freeway, cruising right around 50-55MPH on pretty flat ground. Even taking off 2MPG for error, it's still pretty high for most on this board. 10-13 MPG in the city was the norm, but I'm not easy on it from light to light typically.
I went from 36" tires with 3.73's and getting 9-10 mpg to 39.5's with 4.56's and average 16+ now.
A K5 can get good mileage...but you need a diesel to do it. I got rid of my carb'd gas sucking 350 and swapped in a 6.2 J-code diesel. Never looked back and haven't regreted it for a moment. It's way better on the rocks now too...