Happy Camper - I'm sorry you're having this problem and wish I could offer some guidance, but I just don't know anything about slide-outs/tip-outs in HiLos.
You've got a good thread posted in the the right place here to get some answers, so I deleted the new thread you started in another subforum since we've found that duplicate threads/posts on a common subject can lead to confusing responses that can lead you astray. It's best to have anyone offering solutions to post in one place where everyone with complimentary knowledge will see what is suggested and can add to the information that will help you.
If the wood is sound beyond the rotted area you show, could you attach a metal plate or L-bracket to bridge the gap, perhaps with lag screws? Then, you could possibly fill the gap itself with something like Bondo if needed?
- Jack