Welcome to the forum - do you already have the diesel lump?
If you haven't already, you might want to look at Big Dave's modded Ford F-350 diesel (28 mpg) to get an idea of what's within the realm of possibility for a large, heavy truck.
Details: The Red Baron - 2000 Ford F-350 XLT Fuel Economy - EcoModder.com
fuel log:
2000 Ford F-350 XLT Gas Mileage (The Red Baron) - EcoModder.com
Yours will likely be harder to get to those numbers since its less aero than his, with its tapered "fastback" bed cover. The Suburban drags a much bigger wake behind it, which kills highway mileage.
Also, I'm pretty sure Big Dave has honed his driving techniques to squeeze out those numbers. That's half the battle right there.
Good challenge for sure.