My rubber roof is 20 years old. At about 12 years old a friend bought a new MH with a FG roof and gave me a gal of Koolseal and a gal of primer. I used it because he gave it to me. I had enough for one coat. It looked great for 4 years and it began to slough off. My roof looked like yours and based on my experience I would wait until the black starts to show through the white surface before you coat it. As another data point another friend has a 20 year old Bounder and his roof it totally black the white has all sloughed off but it still doesn't leak. Coating the roof will reflect the sun reducing the inside temps in the summer. Also it becomes the sacrificial coat that will wear preserving your original roof.
