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