If it is a standard full size mattress you need then go to any store that sells mattresses. Several E.O. van owners have had custom mattresses made at The Original Mattress Factory. ATeam has a Select Comfort mattress. That's the way I would go. We have one at home. Very comfortable.
I forgot to mention that the Select Comfort is an air adjustable bed. You will need an inverter to supply 110 VAC. I'm not sure if a 12VDC pump is available.
I have bought four mattresses from the Original Matress Factory which is a chain found in Many cities. St Charles, Mo and Cleveland,OH are two locations I have used. They will make any size, any depth with a vaiety of foams and/or springs. We paid $99 for each of two 2' x 6' mattresses in our present cargo van. Check Original Mattress Factory : Fine Quality Mattresses
i am now using a futon. it almost fills the lth it certainly fills the width. i found that truck matresses were too soft for my back. i also used a pair of egg crate mats.