Hello EcoSD, you have an admirable goal.
Boosting your rig's economy by over 20% is doable, there are other guys on this site who drive trucks comparable to yours and doing as well as that. You're running a diesel, too, which means some things might be a bit easier - forum member Diesel Dave's White Whale
2007 Dodge Ram 2500 ST Quad Cab 2wd, short bed Gas Mileage (White Whale) - EcoModder.com whistles up damned good numbers, and his rig is in about the same weight class as yours.
I totally get not wanting to drill holes. But if you're going the distance of the tonneau, take a peek at some of the aerodynamic bed toppers guys have put on here, including Bondo's "Aerolid," an excellent example of the breed I tend to call a "whaleback." It looks pretty slick.
Aerohead lives and breathes aerodynamics. I think the guy is a literal rocket scientist; if he ain't then he's an aerodynamicist. I dunno, I just know he knows waaaay too much about aerodynamics for a regular person. I view his suggestions as citable resources.
Me, I'm just a guy with a little pickup.
Welcome. It's fun here.