Futon-like Bed Mattress Advice?

I just bought a 2016 Dodge Grand Caravan for camping in. The first planned addition is the bed. I basically want to design a futon. Something for sitting on during the day while parked and converting into a bed at night.

I'd like to have a twin-sized bed, approximately 75" x 38", and would like it to be 4" thick.

My first thought was a futon mattress. The last time I slept on a futon was in my teens in the late 90's, and they sucked then, and reviews of them today tell me they still do.

My next idea is a memory foam mattress, but I don't know how well they would fold vertically.

Another idea I had was two 75 x 20 cushions, but I am worried about the potential to "fall through" the crack in the middle and end up on hard wood.

I am hoping people have tried doing something similar and can advise on the best mattress to use in this situation.

Any advice is welcome and appreciated.