[elhigh]

The skin is aluminum?

I literally take my cap off to you (sorry, no pix). That's a build level we don't see here very often. Coroplast is sturdy and all, but, dang.

I've noticed the gigantic gaping wheelwells with nothing in them for many years now, and the truck designers continue to leave them wide open. You'd think they'd offer the option, just some kind of spats or something for the taller-standing HD trucks.

Skin over that yawning crevasse and you'll be deep into the 20+mpg territory, I'm sure. Airdams, nut adjustment and you can be chasing Big Dave.
[edit]
Whoops, gas engine, maybe not. But still I think you're well on your way.