Mattress Replacement

[Speckul8r]

After 8 years of pretty good use, the mattress is done. Too many hills and valleys to be comfortable anymore. I'd like to replace it with a memory foam mattress but have not found any that I think are thin enough. There is a 4" pillow top/ memory foam combo at Camping World that I'm wondering if it will fit on top of the old mattress when the unit (2720) closed. Closing the rear shell and looking in from the front, it looks like it may make it.

I'm not much interested in a straight foam replacement.

Any suggestions appreciated.