When the truck is pointed downhill, the gas gauge will drop alot. Mine drops about 1/4 tank when I park on a downhill slope.
Tires and alignment have a lot to do with MPG. When my ball joint was bad, I got 16 MPG on an 800 mile highway trip. After the ball joint was replaced and an alignment was done, I was getting 21 MPG on the highway. On average, I get 13-14 around town. I took my chip out the other day because $2 for premium is too much.