Best way to seal leaking storage bins

My son has a Starcraft Bunkhouse trailer. His flooring of his storage bins is at the main level of the floor and is more or less under the main bed. It appears that his storage bins are leaking and the wood floor has rotted behind the bins. There does not appear to be any water in the walls around the bins just the floor itself.

He is in the process of replacing the floor but is unsure how to seal the bins so they don't leak. He is looking for suggestions.

Thank you

Shirley