Rather than fixing a problem I have been on a preventative maintenance program. A few times a year the roof gets washed with the same product & soft brush as the rest of the trailer. The Meguires stuff in a pink bottle at Wally or any automotive store.
Once a year it gets treated with one of the roof cleaner & restorative products. It does not come back looking like new but at 6 years old it looks pretty good & black streaks down the side are minimal. Chalking is non existent.
Six months of indoor storage for the past 4 years helps too I am sure.