If you are thinking it might make the roof noise less in heavy rain, I doubt if it would make a noticeable difference. However, I see no reason not to try it if the price isn't too bad. You could probably use standard 30# roofing felt. It's not going to add much weight (about 60# for the entire roof?).
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