Where to buy an RV mattress like the bunk mattresses?

I think a topper wouldn't work. The problem is that we don't like soft mattresses and adding things on top feels like it would make it even softer (by the way my friend has been buying and returning toppers for some time now and he can't find a solution).

However, I didn't realize Walmart sells memory form *mattresses*. For some reason I thought they only sold toppers. Merrykalia, do you think those foams are firm?