Spider cracks in rear cap

I'm assuming you are talking about the rear fiberglass main cap. Small spider cracks are usually a result from age but are also known to be caused by the fiberglass flexing. If the area is really chalky then I would more so say it was related to age. If not then maybe at some point something bumped the rear corner or even quite possibly the wind when driving is causing the corner to flex. Small cracks like that are usually superficial and can be fixed by sanding and painting that area.

Gelcoat is unreinforced and less flexible than the underlying fiberglass.

As long as they are hairline, they are typically cosmetic. On RV's fiberglass is typically thin so it flexes significantly compared to a boat hull.