Well, it was a long time ago (1990s) but we had a Dodge Caravan that had seats that fold down into the floor to give us a big open back end. IIRC we went tent camping and weather blew up one night so we bailed into the car for the night.
Those floors aren't PERFECTLY flat and smooth (hooks for the seats) and it was stuffy (wet so didn't crack windows).
My preference is up-off-the-ground rather than on the ground/surface. And on the ground/surface depends on if I'm in my Midori 1 with a proper backpacking mattress or in the back of a van with an air float. Now I will say, mattress toppers and memory foams have come a long way in 20-30 years.
No sense making an investment in cots or expensive toppers if this is the only time you might try something like this. I found with a cot I could also slightly elevate the head end which is more comfortable for me when I'm not at home (although in our Aliner I simply use two pillows).
BTW, what month of the fall are you going to attempt this?
Bama Ed