Mine had a roof vent- leaking POS that it was..... I actually removed and screwed a metal patch in its place with butyl tape under.
Fiberglass is no real problem to cut or drill, but whatever you do needs to be backed up/supported with wood or metal framing.
Drilling works best with a hole saw, or in the case of small holes - a stepped bit , or a dull one with slow and gentle speed and pressure. I like to drill screw holes a bit oversize and let the butyl caulking or tape squeeze into the hole to allow for some expansion and contraction
Adding roof racks/ solar panels/etc requires either attachment to the metal sides and bridging the width of the van, or drilling, sealing, bolting directly through the existing roof supports....I would be inclined to make aluminum supports that bolted through the sides and were elevated an inch or so above the fiberglass since aluminum and fiberglass expand and contract at different rates. Also that would allow for future sealing if it became necessary, and ventilation for panels and roof.
Another option might be to cover the entire roof with epdm membrane(rubber) - any through bolting or screws would still warrant using backers and butyl.