I have the same cursed storage bin leak, water damaged floor. I have the same issue with one of the corners on the roof being wet after removing the plastic corner. However I don't see how the water could get in the storage area. I did the water test and it looks like my problem is the white one inch piece of plastic that runs along the bottom between the metal outside front panel and the floor. Water was in between it and the wood was just wicking it up.
I have three sides for the metal frame braces I can put a patch on, but don't want to go into the living space and start removing everything. What thickness of plywood do I need? Do you think I can get a way with a butt joint on the one side, maybe need to seal somehow?