Hi Steven, welcome to the forum! Most folks don't know that RV manufacturers recommend that you check all of the roof and window seals at least 2x per year. It's not uncommon for the caulk to crack and let water seep in, and after a few years, a tiny leak causes what you see here.
These campers are not hard to fix from a materials and skills standpoint, but it has to be a labor of love because it never makes economic sense. As blownstang01 points out, you can see in the pics that the fiberglas is all wavy in front of the front bedroom window indicating the same kind of damage that you are dealing with in the rear bath.
These campers are built from the outside in, so you're looking at taking off the exterior fiberglas and sheathing, replacing the framing, and rebuilding the back wall, part of the floor, and some of the sidewall. Basically keep cutting until all the old wet moldy wood is gone and then rebuild. This also means dealing the with electric and plumbing that runs through the framing, in one pic I see both 12v and 120v running in the back wall.
Add in what's happening up front and you will have hundreds of hours into this project. Only you can decide if it's worth it.
If you look on the A-frame of the trailer tongue, up front by the propane bottles, you should see the 17-digit VIN stamped there somewhere. If you get the VIN and call Jayco customer service, they should be able to give you all the particulars about the trailer.
The trailer looks like it's about 10-15 years old, and bear in mind that appliances like the fridge have an expected lifespan in that range. If you do a web search you will find many projects like yours, I myself am in the middle of rebuilding a slide-in truck camper with extensive water damage. We are happy to help you through the process, but know what you're signing up for...
