I clean mine once or twice a year using liquid Spic and Span, and a soft brush, (notice I said soft)
Then I condition it using a liquid conditioner that I get at the RV shop. There are several different brands, and all you do is spread it on with a sponge or mop.
The whole process is very easy.
Don't worry about black spots if they don't come off. The only ones who will see them are the birds.
Never use a stiff bristle brush!
I am 81 years old and on my third trailer and never have had a leaky roof, so I must be doing something right.
Jack L