Rear floor on a 325 rotten suggestions welcome

[davejay]

Hey all,
I tore out the rear floor of our 325. Today. I don't know where or how it got soaked. The only thing I can come up with, is the edge of the floor is exposed to the elements inside the spare tire compartment. Anybody else have this problem. The gal incised sheet metal is rotten too. It's not a big deal, I'm just curious as to how it happened. This coach has 5420 miles on it. So it isn't exactly road weary . I'm thinking of using 5/8 exterior plywood instead of the partical board that is factory . The water tank is fine. I think it's from the water regulated . That seems to be where the rot is the worst. Thanks dave