I had the same problem.
Our guest sat on the front edge of the mattress with the round black dump valve and inadvertently pushed in the yellow deflation tab inside. The black cap was slightly loose and no matter how hard we pumped the mattress, it deflated slowly at night.
I finally ordered a replacement from Flair (75 bucks for a queen including pump and storage bag - a GREAT deal for an out of warranty item).
Shortly after I ordered it I found the culprit.
Last year I made a puncture and fixed it with a radial bike tire patch using the instructions on the card and it still works great.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW