Proflex RV is a good choice for a fiberglass roof side-seam, but not the only possible choice. There are several quality products, e.g. Sikaflex 221, Dicor, etc.
The reason for avoiding pure silicone is simply that it cannot be over-coated if a future touch-up is needed. The old sealant would have to be removed before re-applying. You can, however, use a silicone-blend sealant, which is typically a mix of silicone and acrylic. If the silicone sealant says it is paintable, it is suitable for use on fiberglass & metal RV surfaces.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL