Kevin,
Our former coach, 2002 Marathon 0740, XLII one slide, never did much of 6.5-6.8. running at 68 mph. It had the DDEC IV. It also ran a bit hotter.
Our current coach, Marathon 0586, 2000 "H", does much better no matter what we tow. Going to Quebec in October, with a tailwind, we got 8.1 mpg. Mostly at 68, then came OHIO's interstates ugh, 55mpg.
Coming home we averaged 7.2 with lots of Illinios and Nebraska crosswind. Total triip was 4700 miles. Final average per DDEC INfO Center was 7.7. The better mileage must have been from the 4 days it took me to wax it.
Jim and Chris
2001 Featherlite Vogue XLV 2 slide with Rivets-current coach, 1999 shell
Previous 22 years,
We have owned every kind of Prevost shell but an H3-40