Best building for vehicle storage question

I would agree. You want some sort of ground vapor barrier.

A heated insulated building would be "best."

Actually "the best" would be to seal the car in a great big airtight box, pull a partial vacuum to get the moisture out, use dessicant, and then flood the container with either nitrogen or CO2, and maintain a bit of pressure above ambient to prevent influx.

Of course, LOL, I don't have that kind of money.