Just replaced ours with this one. The factory mattress was too mushy for my back.
http://www.plushbeds.com/deluxe-foam-rv-mattress-p-101.html
This bed was only around $700 with AAA discount, shipped to our park site.
I was used to a Tempurpedic at home, so I am hard to please. I have to say I am very surprised at how great this bed is, for a fraction of the cost of a T-pedic.
I don't give many testimonials, but after sleeping on this bed for a week, I would give it very high ratings.
We got the 8", 72"x80" and it fits on our BH king platform perfectly. (I believe they make custom sizes, if needed)
I highly recommend this bed, if you like a memory foam mattress.
Pay a ton of money for "custom" if you like, but this bed rates with a Tempurpedic.
Plus there is a 90 day return policy if you don't like it, for some reason.