I put a Wal-Mart Queen size "bed in a box" in my bus. It's a 2 piece foam mattress- I think 3" memory foam on 5" open cell foam, in a dense fabric case. Only used it for one month so far, but the wife and I both really like it. They come in different sizes. Decent foam mattresses tend to be a bit heavy for their size. Using both mattress and box springs will take up space- I built a hinged platform, with storage under the bed. This is pretty common; using both matt/springs you'll either be higher up, or have less storage underneath.
If it isn't grown, it has to be mined