With our current 32-footer, we're averaging in the 7.6 range, including some mountain climbing (there's a lot of that around here!). It's a great deal better than our '85 26-ft Class C with a 460 (carbs). We got 6 mpg at sea level and about 4.5 above 1500 feet.
With all the problems we had to have fixed (manifold leaks, brake problems due to engine-room heat, etc), I estimate our average cost for a day with an overnight, just fuel and amortizing the repair costs for the two seasons we owned it and the bath we took when we sold it, was somewhere north of $400.
It's a minor miracle that that old clunker didn't put us off the RV lifestyle forever!