We purchased from Mattress Firm a full-size memory foam mattress and it's only about 4.5" thick. The mattress that came with our used 2007 255 GTS was a plush mattress and when the slide was closed, it buckled in the middle and I suspect it put pressure on the slide mechanism itself. We've not yet had a chance to sleep on it, but we did notice that it fit quite nicely with the slide closed with no buckling.
Ken & Val [Lexi's mom] Moczygemba & Tori the dog
2015 Forest River FR 3 30DS [Lexi II]