Scott,
The amount of paint required varies a good bit based on your approach, technique etc. But generally a RV8 is like painting 3 cars. Roughly 2 gallons of topcoat, maybe a gallon plus a bit of primer.
These are real rough estimates. A pro painter will have an idea based on experience given the square footage, but for us ametures its more of a wild guess. I almost always have to go for more!