We bought our TT last year and had it delivered to our seasonal site. We leave it at our seasonal site year 'round. After 2 months on site, I went to clean the roof. It had gotten really dirty between delivery and that day. However, it wasn't pure white when we took delivery. I spent hours cleaning it and didn't get it very clean.
Now I just use a soft brush and water. I spend about an hour and remove the surface dirt and don't worry about the stains. This year I cleaned the roof about a week after we opened up for the season and then again a few weeks before we closed it up for the season. I know it's clean, just not pure white. It was actually quite a contrast when I put down eternabond on all of the roof openings.
-Michael