I'm getting 20.5 with a 2003 Dodge Cummins H.O. and FWC Grandby. These are mostly highway miles at 60-65 mph.
The truck is quad cab, long bed, 4WD, 6-speed manual, 3.73 axle.
The FWC is raised a couple inches above the bed/cab, so maybe the MPG could improve by lowering it or adding a fairing.