Section of top of slide rotten

If it were my trailer this would be my approach. First find out where the leak is. If there is room to access the top and front side of the slide from inside the trailer, say 10" to 12", I would try this. Run the slide out about 1/2 or 2/3 of the way. Take off inside trim at top and front of slide. Now remove metal trim strips to get access to roof. You should be able to move slide in and out to access any fastener that are cover up by outside wall. Remove rubber roof using same method. Then remove wood deck by same method of moving slide for access. Now you cam access damage to structure. Replace damaged structure and any paneling on the inside. Cut new roof deck to proper size, using old deck for a template. Slide in from inside of trailer, again fastening in place and move slide in or out to accomplish this. Install new rubber roof and trim with caulk or Eternabond tape in same locations.