Squeezing the best mpg from a RV

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I would not reduce speed much more, you’ll risk being rearended by an inattentive driver. And it wouldn’t make that much of a difference, anyway. 10,500+ lbs is hard to improve much, you’re doing what’s recommended. Make sure the tire pressures are measured when cold (sure you are). There are expensive engine modifications (cam, intake, and exhaust manifold) that might help a little, but you won’t likely save enough on gas to pay for them.