Michael,
The nylon anchors are an option and would be used on thin materials that many other coaches have. I think you will find that the fiberglass on your coach is substantial enough to hold the threaded fasteners of the camera without the nylon inserts which sometimes can pull out. I usually approximate the drill size comparing the bit to the screw thread and drill a trial hole into a piece of wood. For fiberglass work you want the threads to have some bite in the hole but not too much given you can shear the screw or create some small stress cracks in the glass....just enough bite by the thread is enough in fiberglass.
I personally like the rolled sealing material that is clay like that is separated by paper for sealing a mount like this to fiberglass. It is available at any RV parts place. Roll out enough strips to cover the area of the camera mount, press them on enough to have them hold, remove the paper, screw in the camera, some will squeeze out, then with a plastic knife trim the excess clay material from around the camera edges and remove the excess. This clay material does not "cure", stays in place, and seals very well. The plus is that it is easily removed if for some reason the camera has to come off.
I do not like silicone to fiberglass as a seal, it tends to not adhere well with age and it tends to then leak. It is not obvious that a leak is occurring until there is damage. This info is how I would go about doing your project. It is an option and my preferences from having many projects over the years. There is no exactly right and wrong method here, just another approach for your consideration. Hope this helps.
Later Ed
