I've heard of cracks in fiberglass roofs, but never had that problem in mine. If I did, I would simply paint it with an elastomeric roof coating. Much easier than replacing TPO or EPDM, I think.
But I wouldn't let an TPO (Tuf-ply) roof be a show stopper when purchasing an RV. It's an excellent material. We had a TPO roof on another coach and were quite happy with it.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL