I bought one of these on Ebay for my pop-up camper. I put it underneath the camper mattress and it made a huge difference. Its like I am sleeping on my tempurpedic in my bedroom. This is not the exact item I bought, but same thing in a different size. I bought a 3" thick queen camper size. It was a perfect fit. They cut them in several sizes, so make sure you are getting the right size for your bed.
Mattress Foam on Ebay