The key question is whether or not this is an active leak or the result of a previously repaired leak. If it's still active, stopping it is the essential first step.
After I bought my 2002 MH in 2017, I noticed a couple of areas of delamination maybe a foot or so in diameter. I'm not exactly sure of the cause but I had to replace the bathroom skylight which had been leaking for sometime.
Since 2017, these areas shrink in cold temperatures to become almost unnoticeable and reappear in the summer. They're stable, not getting larger and are on the driver's side where they're less noticeable. Given the age of my MH, I've decided they're not worth repairing even though I can access them via the edge of my siding next to a slide.
Clearly, your first task is to ensure there's no active leak. Assuming there's not, I like the idea of a window. Once the hole is cut, you can inject adhesive and will be able to creatively clamp any areas extending beyond the opening. There are a number of Youtube videos on how to do these repairs and having access via the new window will make it much easier:
https://www.google.com/search?q=repa...hrome&ie=UTF-8
I don't see a downside in trying this, especially if the $5k job involves removing and replacing the entire panel back to the slide.
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