We also just got back from a 4500 mile trip, crossing the Rockies twice. We have a TL B+ 213 (with Chevy 350) and decided to take mainly old US Highways and go 60 mph (we even went 60 on the interstates when we used them).
We averaged 14.5 mpg, sometimes even getting nearly 16. I was pleased, but this type of mileage has been consistent when we've gone 60 mph.
We went from Northern Minnesota to Montana, then down through Idaho to Reno, then across Highway 50 to Kansas, then back up through Nebraska and back to Minnesota.