I finally have some number to contribute to this thread that I started!
I recently sold my 2003 Tundra and 1988 FWC Grandby. For what I got for those, I bought a 1999 3/4 Dodge Cummins, QCLB, 5spd AND a '95 Cascade 8.5' hardside camper.
With the old rig I was getting 13mpg. First trip with the new setup and I averaged 15.5mpg. 660 mile trip, hand calc'd, mixed city and highway. First half no faster than 60mph or greater than 1800rpm. Second half, bumped it up to 65mph (2000rpm) on the highway and mostly into a head wind, no change in mpg. Diesel is now 10 cents cheaper than the premium that I was running in my Tundra! Oh yeah, I no longer have to deal with the pop-up!