I have the same problem. What seems to work for me (even if not permanent) is the lysol brand mildew remover. You let it sit after you spray for about 5 minutes or so, scrub and rinse. I also do not have ventilation. I think probably adding a van, that will require going through the ceiling and roof, or adding a window. I don’t have the funds to get what I need yet… but I’ll keep you posted.

Good luck