The tailgate is the culprit. Nothing like a 4-5' wide and 2' high wind blocker going down the road to whack your mileage.
I seem to remember a study posted here a while back that shows that the tailgate does not serve as a wind blocker. In fact the people who are removing their tailgates (or going to the open net type thingies end up getting slightly lower MPG
The closed tailgate forms a pocket of mostly stagnant air in the bed of the truck. There is little exchange between this pocket and the general airstream flowing over the truck. This causes a decreae in air resistance as the truck does not have to pull the amount of air that it normally would.
A semi pulls behind it a large volume of air as the transition from the large square trailer creates a huge drag zone immediately behind it. On a pickup this drag zone is greater with the tailgate open than with it closed.
Referring to the nascar truck picture earlier on the thread, the closed deck over the bed is intended to decreas drag. And it does. However at normal highway speeds the air exchange between the bed and the airstream is not high enough to need a closed bed.
As an experiment you can do at home place a crumpled up ball of paper (or if you live in the country an empty beer can) in the bed and go for a drive. for most of the ride the ball/can will move around some but it won't leave the bed.
I wish I could remember when the study was posted. I'll try to find it.
Please post it and ping me.
My sons had pickups without camper shells and when they added shells, their mileage increased significantly.