The very first thing we did when we got our new 5'er was to take the nasty mattress out of it and replace it with a memory foam mattress that we got from Walmart. We have a Tempurpedic in our house and loved it. Figured we couldn't/wouldn't spent quite that much on a mattress for the RV. Ordered the Spa Sensations 8" memory foam for $200 online, had it in 2 days and unboxed in the bedroom of the RV. (This is a must because it's small until it encounters air and then it is heavy and bulky).
It sleeps as good, if not better, than our mattress at home. Lots of people complain that it is like a chunk of stone if it's cold, but I don't do cold weather camping without heat!!! If it's above 50 degrees, it's fine. The one that we purchased had a cover on it that allowed the foam to breathe and not get too hot. For $200 for a queen size and $300 for the king, when our Tempurpedic dies, we will replace it with the one from Walmart for 1/10 the price.
On edit: The memory foam mattresses require a solid platform, so you don't need, nor do you want a box springs with it.
Here is the one that we purchased - it's gone up a few dollars and is now $219.
Memory Foam Mattress