I wouldn't use silicone cause it is problematic to ever coat anything over it again, including more silicone. There are other excellent products for use on fiberglass, e.g. Geocel Proflex RV, Surebond SB140, Geocel MHRV 2300, Sikaflex, 3M Body Seam Seal, etc.
Besides, failure of the Dicor to stick is more likely inadequate prep work than a problem with the fiberglass. There is, however, known incompatibility of Dicor with some types of plastic, e.,g the acrylic plastics used in skylights. That results in cracks in the plastic, not leaks.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL