Hmmm ... increasing tire diameter has the identical effect to using a gear, or gears, with a taller ratio. For cruising, overdrive does this very thing. When cruising a larger diameter tire merely makes your "overdrive" more "over".
However under non-cruising conditions - where you're accelerating or pulling up a grade - a larger diameter tire could sometimes make your transmission have to stay in one gear lower longer, or make your transmission have to torque-convert ("slip") more.
Since most of the total road hours in an RV are spent under cruising conditions, larger diameter tires should result in overall slightly improved gas mileage.