Actually I am in the middle of repairing rotted floors in my daughter's camper slides.
The first thing you need to do is identify and caulk the source of the water leak. The previous owner may have already done that.
Then make sure that the rotted wood area is completely dry.
Purchase some liquid epoxy like
Git Rot or similar product that is used in boat repair. Follow the instructions.
I also used some aluminium angle to seal the edges. I would post a picture but I'm 200 miles away at the moment.
It can be done. Her dealer wants 4 grand per slide.
Good luck.
Peace
Dave