Design up (2 story) or out
Yes, two story is probably less cost to build because it has a smaller “footprint”, but it’s less than you think: 1) The footings can be smaller because they are supporting one floor and not two, 2) Two story will require additional square footage for circulation (stairs), 3) Because of the height of your RV, you’ll have additional height requirement than normal which will require special scaffolding for construction, 4) Possible special water pump due to extra height of second floor, 5) Additional space required for “Entry” downstairs, and 6) Extra fire rated protection between garage space and living space above. (Fire material is required between garage and living for single story too, but easier to install and less required.)

Maintenance is more for two story: washing windows, paint, cleaning the gutters, etc.

The two story option gives you more privacy, but you’ll have the largest thighs in the county.