What year is the Winnie in question??? Sounds like it has not had the required maintenance for several years.
I did buy mine from a dealer, but got no such "lesson" either. However, the Winnebago owner materials that came with my coach state the roof warranty is for 15 years. AND, in order to keep the warranty in effect, you must have the roof side sealant inspected/repaired either once or twice a year (I don't recall which off hand). I have mine inspected and sealant repaired as required about every 9 months. I don't know if inspection by a qualified RV tech before purchase would have revealed the lack of maintenance or not, but it is likely it would have.
You're right that most of us learn as we go along, particularly if you buy a used rig and get no owner materials or records with it.
All RV roofs require regular inspection/repair of one kind or another. I would much rather have the Winnebago fiberglass roof than a rubber coated roof!
Sorry for your unpleasant experience!!
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Paul (KE5LXU) ...was fulltimin', now parttimin'
'03 Winnebago UA 40e TRADED OFF JUL 2023 / '17 Jeep Grand Cherokee toad