Greetings!
Holes in roofs are one of the worst headaches out there. Sooner or later they always leak, and always at the worst possible times. They have caused more damage, and more moisture & mold problems than anything else.
If you don’t have gutters or a factory roof rack mounting system, and you must have a rack, I would suggest mounting them to the ribs at the top of the side walls rather than the roof. With rain shields around the mounting points, the should never leak, and attaching to the upright ribs should make them much stronger than attaching to the roof ribs.
Something like this that will extend past your fiberglass roof to the metal van sides.
Cheers!
"I can live like a king because I work like a dog." ~ An anonymous vandweller