Unfortunately, you will probably have to work from the top. Not 100% certain with the Swift because it is stick and tin, but with the laminated Jay Feathers the floor is a single unit that is sitting on top of the frame. The frame members get in the way. Also, with the location of your bathroom, I am betting your black tank is directly under the damaged area. It might require dropping the black tank to get access from below.
I did a near identical repair on our X19H. It is about a 3 weekend affair. 1 day of demo to remove rotten material then a few days of drying time for any wet but undamaged material. One day on weekend two to cut and replace the subfloor. Two days to run a new flooring throughout the trailer (actually one day to lay it, but another day to lift cabinets and dinette/sofa and then reinstall when done).
I have a post around here somewhere with pictures, will track it down and post the link. There are others here who have done even more complex and in-depth floor repairs with full picture threads to help guide you.
ETA: Funny, I thought I did a whole thread on it, but just piggy-backed an existing thread. Here are a handful of pictures of what I did...