My $13K 1995 Fleetwood Tioga Montara 23 ft (Ford E-350 with monster 460 big block) driving 55 and 45 on the mountain passes: about 10 mpg
I recently installed a K&N air filter and that seems to have taken it from 8 to 10 mpg.
Fourwheeler.com "Project MPG" series in 1999 has various options for getting more mileage out of a 460 engine... with payback calculations.
My friend's $90K '11 Navion 24 ft with a MB Sprinter turbo diesel: about 14 mpg. Fun to drive up mountain passes, not fun to purchase ;)