Most RV and boat mattresses are constructed of foam and have no springs. In many cases manufacturers use fairly cheap foam. You can improve things quite a bit by going to a foam or upholstery place and buying higher quality, denser foam. You can also buy two thinner foam pieces with different densities. For example, on my boat I replaced the original foam with two layers. The first layer was much denser than the original foam, and the second layer was 1" memory foam. The combination was the same thickness as the original foam; so the original zippered covers slipped right on. The difference in comfort was huge.
On the Hi-Lo my wife and I used a different approach. Since ours has a tip-out, we do not to use the dinette. We leave the dinette converted to a bed and use a memory foam pad on top of the factory cushions. I got the pad at Costco and trimmed the width a bit to fit the dimensions of the dinette bed.