I have a 2016 CHATEAU 23U. By early 2017 my mattress was destroying my back. Long story short, I went with a Foam Factory (https://www.foambymail.com/
) 8" total, MFM, Full-XL, custom cut/with cover to match the original shape for necessary door clearance. I'm still very happy with my choice. Other than being a satisfied customer I do not have anything to do with Foam Factory. I'm just sharing my experience. They are great to work with and they will help with your order to make sure you have the best fit for your needs.
Good luck in your search!