My guess is that the OP has roof-rot. Also, I'm guessing his trusses are rusted or this is a race vehicle and too many people spent time on the roof, which crushed the trusses.
We really don't know.
However, the real challenge of any roof project is how the roof radius is formed.
Mostly, this is grunt work. And you need several young and capable men for this job; and you need people who will not hurt themselves getting the job done!
Note: I also had very good experiences spraying a Rhino Roof on my RV after the wind blew a section of my roof fiberglass off. They quoted me $7,500 before the job started, but the bill was $9,500 when I picked it up.
The Rhino Roof was a home run. The problem was that too many "Joe's" stood in one place on my roof and that crushed as section over my slide-out. So when I extended the slide, that action ripped the vinyl headliner. (What a bummer.)
After that I went to Winnebago to have them replace the whole roof, but I really think the rhino roof would have been an ideal seal and the color was very nice. Likewise, I was looking forward to no more room radius caulking maintenance. I also was thinking there would be some added insulation benefits, and you walk on the Rhino roof without slipping. Plus it looked beautiful! (Oh well.)
I highly recommend all Winnebago roof owners to put a seaonal notation in their maintenance log book regarding roof inspections. For example: "On xx/xx/xxxx I performed a roof inspection found no crack in the roof radius caulk and the rest of the roof sealants were in good shape and meet manufactures requirements for return to service."
If you don't do something like this, then your insurance company will 90% NOT PAY you for wind damage to your roof.
Also, in my case, I went the FAA website to find the rated wind speed at the local airport... to prove gusts in the area I was traveling was over 50MPH. And I took pictures of the news reports on TV to prove wind gusts were not normal and caused my roof to be damaged.