Many toppers are vinyl, so not much to do with them. They are waterprrof as lng as the vinyl is intact.
If woven fabric, they may be Sunbrella, some sort of canvas, or a woven acrylic fabric. Most any of the woven ones can benefit from something like 303 Fabric Guard, and the material was treated at the factory when it was made. If they aren't leaking yet, no worries.
However, if your concern is cosmetic, you need to get them clean first. Coating over dirt just makes the dirt waterproof.
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