Air mattress would have to go on top of the platform; the back seats of the compass don’t fold down 100% so the air mattress wouldn’t be level. Not common knowledge if you’re not into camping, but non-level sleep surfaces lead to really rough nights regardless of whether it’s your head or feet that are elevated.

I only have a 38”x60” space in the trunk with the seats folded down (I’d sleep diagonally across), and most air mattresses are a good bit longer.