hard bed

Memory foam topper - many purchase from overstock.com. Even if you have odd sized bed you can cut the foam to size - makes a world of difference and doesn't cost an arm/leg.

Putting box springs under a mattress would make it either very HIGH from the floor or would mean loosing that valuable under bed storage. OFTEN it might also mean relocating other items sometimes located under the bed (like water bypass systems, water filters etc). There are some quite comfortable RV mattresses and if the better ones are not comfortable enough they are often used with a 2" memory foam topper. My mattress in the RV is as comfortable as my mattress at home but comfort is a personal matter. You'd need to look around for the solution that suits YOUR OWN comfort.
Good luck / Skip

Instead of band aida, why not simply buy a quality mattress from your local store? Seems like a better use of the money to me. We installed the same version of our home mattress in the trailer and could not be happier.

The mattress is too hard, and yet you're able to feel the uneven contours underneath? I agree with CincyGus that a memory foam might work.
I have a thin 1.5" memory foam topper that I put into a duvet cover, and it transforms my generic mattress cushions into a dream sleeper.
Good luck.

I've not found a RV matress yet that was to my liking. The Serta's that are normal home thickness seem hard. Our little TT came with a little thin matress that is about 3" think and just slightly softer than the plywood it lays on. We bout a 3" memory foam pad from Sam's club and put on top of it and used it all last year. Was so comfortable, we took the memory foam pad out of the camper after we finished last year and moved it to our home bed.

Last weekend, I went and bought two more, one for the camper bed and one to slice in half for the two bunks in the rear my daughter and her friends use. Now we all will sleep in comfort.

These ran about $100 each and for us, were money well spent. They don't get hot, they relief the pressure point pain we were waking with prior to having them and we are sold on them.