I thought it might be helpful to post some pictures of the process of removing a window in a Trillium
The first picture is of the plastic strip that held in the rear window along the bottom. It was held in with rivets on the inside under the window. We were able to drill out the rivets before the removal of the window, and then used screws to reinstall it after replacing the wood.
The next ones show the removal of the window using a long handled screw driver. Once the screws are removed from the window, the wood on the inside is free to shift to the side so that it can be moved out of the way of the window. So don't pry to hard between the window and the wood. Make sure you have someone to catch the window!
Once the window is out you can see the wood and check it for rot and moisture.
I was very fortunate and found a sales person who was able to cut the plywood to the shape I needed, and then I cut it to length. I purchased half a sheet of plywood cut so that it was 8 feet long and 2 feet wide and then ripped into strips with one edge slopped. I did look into using pressure treated plywood, but I could not find any dry sheets, so I went with regular plywood. I took one of the old ones with me when I went shopping.
We used a double thickness of "putty" when we reinstalled it and then finished off the outside with Mono exterior caulk. We did not caulk the bottom edge of the window as we were advised it needed to be free to drain.
job was chipped a bit, but once the Mono was on, it hid most of the chips. We will now touch up the paint
the mono so that dirt etc does not stick to it.
Hope these pictures help others understand what is going on inside the window frame.