I was all set to order a custom mattress but decided to go in a different direction. We love our latex mattress and really wanted to have the same for the RV but the most we could find was a mattress that had a foam base with a latex 1-2" layer at the top.
I had nothing to lose so I decided to order the following toppers
One is a 3" firm and the other is a 1" medium. I then cut the existing base and top mattress width-wise and pulled out the existing foam. The top mattress was a bit of a pain as it was sewn in several spots. Next, I had a zipper from the link above sewn into both pieces. Now all I had to do was add both layers of latex into the covers and I was done. The mattress looks factory but it's way more comfortable than it was. It's also heavier as latex is not lightweight so I give the bed a little lift as I'm putting it up to compensate for the weight.