Man, I'm in it thick myself with very, VERY, similar problems, literally at this exact moment... so I feel your pain, and I'm happy to do Facebook chats, or whatever to help share the experience (Its nice having perspective, especially with one that is literally in the same sinking boat!).
Mine started with roughly the same symptom. We decided to install bunks in the slideout where the dinette is, and move the dinette to the passenger side, forward of the galley. We have the 395 model, so we have a lot of space, so it just made good sense to us.
SO, I removed the dinette (no problem), built the bunks (no problem), and went to start planning the dinette in front of the galley (insert beginning of the problem). I noticed the interior wall in front of the kitchen base cabinet had a little bubble. I pushed on it and it was mush.
I peeled the paper back, it was black.
Then I followed the softness, and you may have guessed... behind the cabinets
. So we removed the entire kitchen to get to the water damage.
When I had full access to the damage under the window(s) (thinking it was seals gone bad on the galley and forward windows), I started working my way up... and up... and up... The lap seal bead had failed between the roof covering and the rain gutter. And water was just cascading down the inside of the wall.
So, we are remodeling our coach!
Literally, because the damage is so much and we had to pull the kitchen cabinets out, we're using it as an opportunity to remodel.
Now what to expect --
You are correct- there is 1/8" or 1/4" plywood with a factory-applied covering over it that is adhered to the styofoam insulation. That insulation is then (from what I can tell) adhered to the outside shell. I would not be inclined to remove the styrofoam, if you can prevent it.
Instead, what I've done-- I've peeled and crumbled off all of the old plywood as far as it is soft, and even a little more, all the way down to the tiny layer that is glued to the styrofoam. From that point, run a fan and a dehumidifier on it. I found that in a day with the dehumidifier, it looked significantly better already.
Styrofoam is practically waterproof-- the physical qualities of the foam make it so water generally beads off and goes somewhere else... like wood. So, thats where you are in luck... because you should be able to just peel off as much as that water damage as possible and find some sort of fix. For us, we are remodeling the entire kitchen, and stripping all of our cabinets to repaint and update the coach. So we're in VERY deep in our project.
I seriously thought this was going to be an easy job... build bunks, move dinette. Now we're gutting the bus and remodeling it!
So after you peel everything back and allow it to dry, get a quart of Kilz for safe measure and that whole area. This is why I found it particularly useful to leave that little bit of what was left glued onto the styrofoam.... you will have a surface to apply the Kilz (liberally).
I found myself to be pretty certain that the water damage attacked the plywood, and did not go down the exterior sidewall, because the foam is still VERY well secured to the outside wall. This meant to me that the water simply found the best route downward to create as much destruction as possible, was to let the wood absorb it, and do its dirty little ninja-like deed.
So I'm going to shamelessly add photos... but also note that I did inspect the frame and did not find anything OVERLY concerning. There is some superficial rust, and Im going to take care of that. But I'm going to the RV shop today to get new Dicor and I'm gonna do a reseal of the lap seal around the roof, and re-seal the windows. (Mind you, Im thinking I will have to remove the galley window in order to re-seat, re-secure, and re-seal that window.)
I hope you found this helpful...I do all of the work to my coach by myself, and I always aim at learning 100% of everything about it so I know. This job has already taught me a lot. But, as a contractor, and my wife being my designer- we're coming up with some really cool ideas to update our coach with this remodel!
Let me know if you want to connect on Facebook or something so we can keep in touch. I do that with Dave and Linda through here also. He also does most of his work on his Tradewinds. They are extremely nice people. -- I love our Tradewinds community. I love the coach, and the people are always willing to help each other.
I'm in pretty dire straits with my coach right now. Had I not built these bunk beds, I may have never known the extent of this damage... so... I'll count that as a blessing?
Take care and good luck!