Hey aaadave. We found the same problem about 4 weeks ago with our dining room slide, in our 2003 3655 FL. My husband Steve left a thread regarding how to change out the floor. I also started a post titled "rotting out dining room slide", listed under the repair section, on Aug 28th. If you have any more questions, PM us and we will tell you more about our experience. We think the damage came from the outside slide trim, as the rot was more severe on the bottom of the plywood floor. That trim piece should be recaulked after your floor is replaced. Good luck!
PS: When you put the replacement in, it is much easier with 3 people, 2 to lift the slide on either side, and one to slide the plywood into place.
Steve and Tara Krumm
2004 GMC 4X4 Duramax Allison
2003 Montana 3655 FL