I think that is a good idea to remove the window replace the water damaged interior and that way you can see the extent of the damage once the damaged part of the interior wall is removed. You said it was in your slide? By any chance is the wall behind a sofa or other piece of furniture that if you cannot get an exact match, the furniture could hide the replaced wall section? I think when you go to silicone seal the window when you go to replace it, you may find that debris from manufacture could have caused a breech in the seal and allowed water in maybe during washing or hard rain. I had a friend that had the same problem in their bedroom slide, and when they pulled the window out, they found wall board debris in the groove of the window that had been sealed at the factory causing a breech. They caught their leak early before any real damage was done to any interior areas. Good luck. I will watch for your post and see how you made out.