The biggest effect on your milage is the engine rpm not mph. Your rig has a rpm "sweet spot" depending on horsepower & torque curves converge. In order to get the best mileage, you need to maintain that sweet spot rpm all the time. My Class A DP has 400 hp & 1200 fps they are both @ 1700 rpm & on the flats with 0 wind, that usually equals 67mph. Hills, valleys & curves effect the sweet spot (SS) rpm tremendously, as well as engine temperature & tire pressure which can throw your mileage all over the place. Tires are another gas guzzler. If you have too little pressure, it will take your millage down and cause the tire temp to rise. Your tires age & condition are the most important thing you can regulate & they effect your safety & mileage. Just make sure you air up your tires to the correct pressure for the rigs LOADED weight of each tire(s)! My coach tires are kept at 115 lbs cold pressure all around for my weight on each corner. When they warm up after driving some, they go to 125 lbs warm usually 10 degrees more than ambient air temp outside. This IS the most important area to focus on is proper tire inflation & age, generally, no older than 6 years old, no matter than mileage on them.
My coach has new tires, & I got 5.9 going west on I-40 with a 50 mph head wind, between Gallup & Flagstaff recently & on the return trip 7.2 on that same stretch with a small tail wind. Average speed in both directions was 58 mph, no stops. I kept my rpm as close to 1700 rpm as possible by manually shifting the tranny when it would lug down on hills pulling my '13 Avalanche behind a 40 footer. I have had as high as 9 mpg on I-10out west Texas & as low as 4.9 in Colorado. Going faster than your SS rpm just guzzles the fuel, and most often you don't gain any time. Look at your engine/tranny combo specs for hp & torque sweet spots & try some rpm that gets the best of both. All depends on your personal preference as to time, driving comfort level & money. Usually you'll get 2 of the 3, but not all 3. When you do get all 3, you'll have had a great driving day. It varies all the time. That's why you see truckers driving the big rigs in the 65-70 range, they know that SS for their rigs, & when you see some doing over 70 as high as 80, either they're drinking fuel to save time or they have gear ratios to accommodate speed & mileage on the flats. You'll pass them on the hills. If I push my rig into the 70's (not confessing to more, here) the mileage goes to pot, under 5 mpg. I work hard with the cruise control set for 67 to 70 on the flats @ 1700 to 1800 rpm = that mph range consistantly. One side note, my engine side radiator cooling system on the DP is hydraulic not belt driven like yours, so the SS rpm of 1700 keeps the hydraulic pressure just right for engine temp!
Sorry for the long post on mileage. But there are many variables, more pronounced in a rig, than your car.
