Welcome Michael!
Congrats on your new Sunny! Sounds like you have a project on your hands. You have come to the right place to get help on your Sunline. We have has several total camper restores and some with minor repairs. Do not know which size your restore is just yet.
I'll address the easier questions first,
On the door frame, and then you said wood rot. Not sure if the wood rot is in the door or somewhere else. Assuming it is in the door, many have totally rebuilt the door on your vintage Sunline door. The wood rots out inside the door and it then starts falling apart.
Here is a picture post by club member HandiJohn1 on how he did his door rebuild. We have had many others rebuild them too.
https://www.sunlineclub.com/forums/f71/t2262-door-rebuild-14425.html
The window in the back. That seems odd they took the whole window, but one never knows. I wonder if they had a small AC unit stuffed in the hole?
What size is the rough opening in the camper? We can help search some of the places we have run into if they have any that small. Ebay also has some by sellers that can be cost effective verses having one made up to order. The harder part is going to be, you need a square corner window. They are available still, just not as popular as the newer round corner windows.
Now to the rot, if that was not directed to the entry door, where is the rot you are referring too? Need some more info on this to help better. If this rot is in a wall, ceiling or floor, lets talk first on how to best inspect first how big the damage is, then how to take the camper apart so the repair is simpler and you try to save the inside as much as possible.
Also, please posts some pics of the water infected area to help show us what you are up against.
Hope this helps
John