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I went down this road once myself. I have a 1997 27' Fleetwood Jamboree, E350 with 6.8 V-10 Triton. Now 25,000 miles. Best mileage is driving under 60 mph at about 9 mpg. Against the wind?, well if you have to know, about 7 mpg. That is fairly well loaded at about 14,000 pounds. Good power and acceleration though. Valvoline durablend in the engine improved the mileage.
I have a 1999 Itasca Spirit 31 ft with a 6.8 liter V-10 with 27k miles, most of it towing a dingy. I get approximately 8 mpg towing a 1999 Malibu at 60 mph. Without the dingy, the mileage improves but I do not have data.