We too have issues with the mattress in our 2013 24S. I've been looking around and found this place.
www.finewebstores.com Once on the site put in a search for motorhome mattress. Apparently you can have your mattress custom made but are more expensive than the BB&B, WalMart, Costco, etc...options. We have not pulled the trigger on a new mattress as my wife says the OEM mattress is not too bad. We are going on a month or two trip this summer...we shall see then. Good luck