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Not really sure you can online........but, I can direct you if you can attach a few photos of that area to your post.
You want to use Dicor 501 Roof sealer for anything on the roof or seams. You can get 1/4" Luan at the lumber yard and the same with any insulation. You can get some new plastic or fiberglass ceiling panel material from most RV places. You can buy it by the lineal foot and 8 ft. widths. Kind of spendy and it comes in various thicknesses and finishes. Hope that proves helpful. Please get back to me if you need more input and please take the time to enter a rating on my help to you on this question as that is the only way I receive credit from the site for my assistance. Thanks!
Let me see if there are any videos that might be helpful. All these are nearly the same as to how it is done, but not exactly. I think a phone chat would be more productive here. No extra charge for multiple calls either. You decide if you think that might work for you.
If you Google something like "Fleetwood Popup (or model) roof repair video" there are quite a few out there. Check them out and see if one fits your situation pretty well. You might even try another brand and get a good one.
Not sure you will find that here. You may need to go to a repair facility that actually works on this make and model. That might be tough. I will opt out of this and allow another tech here to offer something if they can. You can always request a refund. We can resolve most issues on this site, but some are just too complex to do in this format. You can't find a perfect substitute for "hands on" all the time. Good luck with your project!