Welcome aboard, this site is fairly quiet, but I'm glad you found us.
I am building a similar project, hoping for 10plus mpg, my truck is a 2001 GMC 6500, with a 210 hp 3126 cat and a 6 spd man trans 3.07 rear gear with 19.5 low-pro tires runs down the road 80mph with ease, I don't know mpg though.
There is an expediter truckers website, they would know that truck design inside and out. might want to Google them.
Tell us more about your truck, gears? tire size? and engine hp. These trucks eat up a lot of hp (and the 3126 isn't a 500hp Turbo Caterpillar engine). When you build big tall boxes that catch wind expect to pay at the pump.
How far do you plan on traveling and what kind of 8000lbs are you towing? Some boats pull easier then enclosed car trailers and horse trailers. Again, wind drag means so much.
Are you building a new box or fitting out the expediter box? check out the photo section here, many nights of idea shopping there.
good luck, keep us informed, perhaps we can help each other out here.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio