self inflating single
I bought a self inflating mattress at Costco 2 years ago, single width, durable nylon cover. Thermarest knockoff.
I use it on top of a carpet kit w/padded foam, on a platform that I've modified, to fit on driver side vs. full width. I have a tall Callen shell, so I have more headroom for an elevated platform vs. simple shell, and storage underneath.
The air mattress + foam= added comfort/warmth. (+Just saw them there again today for about $35. )
I also carry an old super lightweight backpacking, closed foam pad- the bright blue object seen in the photo. I throw it on the ground for lounging, or use it under the air pad, if I want to sleep outside and prevent rocks from puncturing the air mattress.
Or, Costco also has a queen size air mattress with pump. Seems a bit less durable but if you need the larger size, the mattress + pump combo was $40.
Just bring some padding to use underneath you with an air vs. foam mattress- the air can get chilly.