No sagging on ours. There are some mods that people have done to make sure there isn't any sagging. We plan to beef up the supports at some point; just a minor fix.
Also, the memory foam mattress we purchased from amazon.com for our 19 ft.was a Foamex 8" Eco memory foam mattress, queen size, free shipping, about $330. We are happy with it as it isn't too firm, not too soft. "Just right," as Goldilocks once said!
Link Here: http://www.amazon.com/Foamex-Eco-Fri...9479118&sr=8-1