That's a U-haul
The ones I've seen have a couch cushion that's only about 14" wide (or so) when the couch back is flopped down. That is the concept Pat is asking about, but I thought the U-haul seat would be a bit too narrow with the back flopped down like that.
Edited to add: Here is a photo of what I am 100% sure is a U-haul 13 front couch/bunk. You can see here why I'm saying it looks like it would be a bit tight to have a dinette there with
the couch back in place:
I was saying that maybe you could do it with an unupholstered couch back (essentially a piece of plywood shaped like the couch back without it's padding and fabric) that you could add a pad to when it was flipped up in bunk mode.
Pete: The Thermarest is an inflatable pad. It has a little bit of foam inside which makes it self inflating. Depending on which model you get, they are about 1" to 2" thick when inflated, and deflated roll up to tube (like you roll a sleeping bag but much smaller in tube diameter). They are very comfortable. Not that an air mattress wouldn't work also.