Our coach is a 2004 Marquis Sapphire, 45 feet, and weighs about 43,000 pounds ready to go. We pull a Chevy Avalanche which weighs about 6,500 pounds. Our engine is the 505 hp, Cat C-12. I keep track of the fuel gallons, fuel price, and miles for every fill in a ledger. Over about 30,000 miles I think we average between 5 and 5.5 mpg. The highest mileage I have calculated on a fill was about 6.8 mpg, the lowest about 3.8 mpg. On fast wide-open highways with high speed-limits, I have set the cruise control as high as 78 mph. The best mileage (6.8 mpg) I have seen was between Butte, MT going south on I-15 to Rexburg, ID, then across US-20 to the Boise Stage Stop near Boise, ID. Why? There must have been a tailwind, eh? We rarely travel more than 500 miles on a fill; the tank is 150 gallons.
On a March trip home to Oregon from Arizona a few years ago, we traveled with another couple from Scotty's Junction, NV to Mountain Home, ID. I followed at their speed of 62 mph. We took the back road from Tonapah, NV to Battle Mountain, NV - in 200 miles we saw barely a handful of vehicles. I don't remember seeing a significant increase in mileage for that tank.