We replaced the original, thin, mattress-like object with a Cal King memory foam mattress from Mattress Firm. The width fits perfectly but I extended the bottom (hinged portion) of the bed support. Slide fits right past it.
I'll admit that it is heavy when trying to access the storage so I fixed the hinged portion of the bed support by screwing it to the bed frame for rigity and then cut and framed-in a new access to access that space.
We just camped for a week in it and it was great. Easy access to (4) crates we store in there.