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Been watching for an answer to your question. I think you would stand a better chance of the answer if you gave a little more information, i.e. year and size of the RV.
I have a 1995 28' Shasta on a Ford E350 chassis and it has a 460 engine.
My milelage averages 8 to 9 mpg. Limit highway speeds to about 60 mph usually results in 9 mpg range.