The white top layer of the roof is cosmetic - the waterproof part is the rubber backing layer. As long as that is intact and the seams sealed, it doesn't leak. Coating a 20 yo roof is proactive - it's gonna develop a leak sometime. If you are worried, then now is the time to do it. With the product of your choice.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC