It's weird how that works: way back I had an '85 Chevy C20, carbureted 454/400, got about 7 MPG no matter what speed or load.
The '89 GMC Dually I've had for a long time, FI 454/400/GV, gets 10-11 MPG whether it's loaded up & towing, or empty. (Pre-GV, it got 7-8 MPG.)
Before oxygenated & ethanol gas, I once threw everything I could think of at it, and on a steady freeway Cruise, just made 12 empty.
Good thing is it doesn't care about octane, thank God.

Jones, I'd be curious what your rig would get with more timing and octane.