Had the same problem with my 2015 trailer for my Z8. I peeled the non-skid back and replaced all the bolts with hex head stainless with locking nuts. Before I ran the new fasteners, I filled the existing holes with Marine Tex keep water from entering the void between the two fiberglass surfaces. I also used large washers to give more surface area support for the bolts. See pics from after I replaced the new bolts. I then cut a round hole into the non-skid where the new bolts align and re-gluged the non-skid. Those pics I don't have.
Fender 1.jpgFender 2.jpg