Chimo found a really interesting option. That said, although expensive, you won't go wrong in terms of sleeping comfort with what you get from Yachtbedding. Shannon does a first class job. We had the latex topper on our R-27 and now have one on our R-31. Can't say enough good things about it. Yes, it is heavy. And yes, it is makes it less easy to quickly get in and out of the underneath storage areas. But we sleep-oh-so-comfortably-and-well. And that is what we notice most.
Shannon ought to be able to give you good advice on memory foam versus latex. We have a latex mattress at home as well, and it came with four layers in three firmnesses (you adjust the order of the layers to achieve a personalized comfort firmness). Shannon can tell you if she offers firmness options on the latex.
Bottom line: a topper of some kind tends to be a huge improvement to the comfort of the factory bed. Yachtbedding is one excellent (but again expensive) source. There are others, and sometimes some very good local ones.
Good luck!
Gini