I work with vinyl every day and my experience has been that the vinyls that were available 20 years ago were better than what is made today. Go back 40 years and it was even better, so that might explain why it's lasted this long. Our Sunline sits out in the backyard getting all of the sun and weather, but since it is rolled up most of the time, the sun really isn't too much of an issue (except for, as you said, the top 12" which is always exposed), it's the water that seeps in and causes stains. I tried to block the infiltration of water, but didn't have much success. That's why I'm trying to find out if maybe an awning fabric might be a better alternative.
1992 T-2470 (now residing in South Carolina)
Current Sunline-2007 Solaris 2499
2018 Silverado 1500 w/Max Trailering pkg.