I know that this thread is a bit dated but I wanted to add a bit of follow up regarding the memory foam mattress in the RV.
I looked at the gel foam toppers at Walmart and after spending a great deal of time researching and reading reviews I bought an 8" gel memory foam mattress from Walmart and put it in my little 5er.
I then stayed in that trailer for almost 4 months including a month in Mexico.
When I go home I bought 2 more of the same mattress. We are sold on this mattress with two minor issues.
First: The mattresses are noticeably different in feel from each other. All comfortable but not consistent.
Second: I didn't know that you need a ventilated base for these mattresses and had mildew issues on the bottom of the one in my big 5er.