[Big Dave]

Not hopeless at all. You can improve your MPG by 25-40% easily enough.

I recommend a "fastback" sloping bed fairing. Mine was worth 3 MPG over an open bed.

Aero improvement beyond the bed fairing is not gonna be easy or effective.

There is one mechanical thing you can do. GM diesels eat automatic transmissions. Next time it breaks, don't replace it. Convert it to a NV4500 manual. Not only do you get markedly more efficient by losing the torque converter, but you gain a better overdrive ratio and a lot more durability. A NV4500 will easily gain you another 2-3 MPG over and above what the "fastback" cover will gain you.

The mileage you drive makes more aggressive modding worthwhile.