Ugh wet wood in Bigfoot

Without access to the back, it will be difficult to make a durable repair. People say these are built like boats but they are not really. The core is plastic and does not rot (though it can degrade and delaminate from the skins), the inner skin is luan plywood which is what rots. Pushing epoxy through the holes from the outside will not fix the luan on the other side of the core, it'll just make a mess. It might do some limited good for the plywood backing the jack area, but you'd be doing it blind and would never know. On a boat, you would cut a piece of the outer skin off, fix what's underneath, then splice the piece back on, that isn't that much work but then finishing it to look good is a lot of work. I think I might be tempted to cut a nice neat hole in the shower, do what needs doing, then put a nice neat access panel over the hole. That will look OK if you do it carefully. Use 1/8" G10 fiberglass, round the corners, paint it the same color. Could be pretty big, I'd start with some smaller exploratory holes to see that I was in the right place and then extend to where I had to go.