Thanks. Looks like it's another trip back to Menards... I'll go with the "outdoor rated" non-treated 3/4" T&G.
The polyiso stuff is rated at 16PSI. If the 15PSI polystyrene is holding up, this should, too. I won't have any point loads (beyond my own footsteps). I'm trying to minimize framing, but I suppose I could put a sleeper down the middle of the heavy traffic area. All furniture (dinette, cabinetry, bed, etc.) will be framed so no point loads to worry about there.
Thanks again, y'all! I love this website!