Pretty much anything on a gasser 1/2 ton is going to relegate you to about 10MPG. It's mostly about aerodynamics than anything. That gap between the roof of the truck and the cabover is the worst offender, and the flat rear of the camper is a close second.
Your best bet would be a pre-2007 3/4 ton diesel with a lightweight popup, and some aerodynamic tweaks. Someone here has a design for a baffle that fits between the cab and cabover which makes a huge difference, and you can use any number of tricks on the rear to improve aerodynamics.