When you talk about debris being drug in, I'm envisioning you're getting fragments of that OSB that is clearly water damaged and long-deteriorated underbelly fabric. I guess my issue is that you obviously are still getting water in there as evidenced on the, ummmm, "dealer repair." I'm a guy that likes to do a proper repair once over band-aiding it unless absolutely necessary. Big part of your problem on that corner is that the OSB has expanded it making it "fatter" than it was supposed to be to clear as it comes in. The end of the OSB is also exposed, and that's not how that usually is. The thing about underbelly tape is that it is designed to stick to the fabric and not really wood so you'll likely peel it right off first retraction! I think if I had to band-aid it, I'd bend up some aluminum flashing that I could cover the exposed end of the OSB and span over the torn underbelly fabric This would also give me a good line to caulk to seal too. Just strikes me that slide-out may have seen repair even before the dealer did what they did. I would also suggest you get some flexible PVC shower liner (Home Depot or Lowes) or some very flexible, thin splash guards material from your local autoparts store. At the bottom of the slide seal at the trailer wall at either end, make yourself a deflector to keep water from "rolling under" if that makes sense.
