Poor gas milage

I bought it at a Gm dealership. It had a complete tune up. Fuel filter, cap and rotor, new wires, new spark plugs, set to specs ( I pulled them and checked them myself ) new air filter, The lifters were checked and set to specs. The timing was checked and set to specs if needed. I also saw the work order for it. The truck has a throttle body on it. I was wondering if that had something to do with it or one of the sensors. I can normaly find the problem. But this is a stinker.
Thanks.

I made a trip to Indiana by South Bend I left Murray Ky and I had to fill up at the outskirts of Indinaplis I averaged 16 miles per gallon. I don't remember how many gallons of gas I put in. That was about 320 miles I drove I had another 1997 GMC 4 wheel dr and got 19 to a gallon. I know it can do better. The best I got with my old truck was 20.5. I will have to check the TBI and see how many ports it has