It's really weird how milage differs between folks... This is what i've seen myself, as i've been in these late model pu's on trips.
My friend in NH's full V8 toyota got 18 mpg, when him and i went on a trip.
Two neighbors of mine are brothers, and live together. One bought a 2009 Chevy 1/2 ton with a 5.3 V8, and it will get 20 on a trip. The other has a 2008 3/4 ton with a 6.0 V8. It gets 18mpg on a trip if he holds it to 70mph or less.
My other neighbor has a 1/2 ton with a small V8, don't know if it's a 5.3 or the smaller one, but i've seen it get 19 with a pretty big load of bale spears in the bed. They were below the bed sides, but quite heavy.
Everyone has a story, but the above i saw myself.
Another friend has a late model Chevy 1/2 ton with a V8. (don't know what one) He was just in SD on his annual pheasant hunt, and he said he got 20 mpg on the round trip... I believe him, as he has always been honest about gas milage with me in the past.
DM