[Wanderman]

More info. It definately appears that the water ingress is from outside the coach. Likely the seam between the rear cap and the body as well as the weatherstripping that was not attached at the top of the horizontal engine bay access door.

The rotten wood was almost a perfect diagonal line from front right of the bay door to the back left of the engine bay ceiling. I initially thought it was limited to the bottom of the wood/foam/wood sandwich, but it was both top and bottom, with some on the inner body wood (no idea why that's actually glued there???)

After removal I ran the tub/shower and toilet a lot and there is no leaking water from there at all.

I was told by Newell that the wood was marine plywood 1/2" each piece with 1" foam board in the center. You can see by the pictures this is not the case as it's obviously OSB. It sits on a squared "C" channel all the way around with vertical pieces of plywood shaped into a box to support the tub and tub surround above. It appears that it was assembled before the floor above was laid down. I am hoping I can finagle the new pieces in there somehow. I still have to get some of the non-rotted wood out of the back slot, as it's in there tightly with some adhesive on the vinyl fabric making it very tight.

I will be using a water resistant 1/2" actual plywood and will probably put some non-flammable paint on the outer two sides (top and bottom) as the foam board is glued to the inner faces. I'll use some additional paralastic adhesive to stick the vertical tub support box pieces above to the tub bottom. And then some expanding foam to fill the voids around. Lots of those, likely for where the jacuzzi machinery would have been if my coach had that option.

Then the rest of the layers are attached to the underside plywood with an aluminum angle frame and screwed to both the underside of the steel "C" as well as long wood screws and fender washers at various places to hold the diamond stick cloth blanket in place.