[Big Dave]

OK. Where to start?

I don't think 20 MPG is in the cards unless drive at speeds that would get you shot/arrested on the Grapevine. But you can get improvements.

At 5 MPG I'd bet you are wildly overgeared. Probably a 4.56:1 axle. For that truck maybe you should think 3.73:1.

Check your brakes. Sintered metallic brakes rust on the edges and swell up, causing drag. Easy to check. Jack up the front end and give a wheel a spin. If it spins easily, that isn't your problem. If it stops in less than 2 revolutions remove the brake pads and use a grinder to chamfer off the edges. If one drags, probably all four drag.

The wing thing on the cab is effective. You need to fair in the undercarraige, especially under the cab. A proper boattail might be 590 feet long.

If you don't want to come down long grades like a Dauntless at Midway, a US Gear exhaust brake will help a bunch.