We looked into foam mattresses, but we plan to use or conversion primarily in summertime, and foam mattresses hold heat where a coil spring will not.
Ours is a "Full" as we needed the small difference down the sides from a queen. it is only 4" thick. they can adjust the firmness/softness of it by the choice of coils that they use. the coils are 'bout an 1-1/2 in diameter and in a material sleeve then the sleeves are all sewn together to form the support, and then installed into a standard type padded mattress cover.
Really if you are going to spend a lot of time on it, do it right, nothing beats getting out of bed fresh and ready to take on the day. Besides, I have a hard enough time sleeping while out on the road, new noises every night, RV parks are usually close to a hiway, screaming kids in the campground, loud party's, new light night arrivals or early departures. etc. I figure why have the mattress contribute to the problem.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio