After having a tube style water bed, I would never, ever wish to go back to any "spring" type mattresses. No matter how much "padding" they add, you will always feel the springs and any movement by your partner.
For our RV, we went with a custom made foam mattress which is built in several layers. Each layer has a specific density and is topped with a layer of memory foam.
Be aware with memory foam, it traps your body heat too much, newer versions of memory foam with blue cooling jel does help that some.
Be aware with memory foam, you will sink into it, it is soft so if you need firm you may wish to consider latex foam as it can be found in several different densities and does not trap body heat.
For custom made foam mattresses we bought ours from
http://www.thefoamfactory.com/ which was cheaper at that time than buying through normal mattress companies..
They vacuum pack the mattress and ship it via mail, you pull it out of box in a place large enough to fit the entire size and remove the plastic and it will expand out.
Another one I can think of is
https://www.casper-mattresses.com/ , never tried it though.