An antenna ground plane is basically a reflector for the signal being broadcast from the antenna. Some types of antennas need a ground plane while others do not. Fiberglass makes a crappy ground plane because the radio waves penetrate it. They don't penetrate metal, so metal vehicles don't have ground plane problems.
Antennas designed to be mounted on flat surfaces (e.g. magnetic mounts, roof top mounts, rear deck mounts, etc.) generally need ground planes. Other types probably do not. If yoiu get an antenna designed to mount on a mirror arm, for example, it won't have a relecting surface under it anyway, so the antenna design probably includes a built-in wave reflector/propagator.
They make "no ground pane" antennas for mounting on the surfaces of fiberglass boats and RV. But if your antenna is mounted on a metal surface (you can even glue a small square of metal to the fiberglass if you like) it will work fine anyway. Fiberglass elsewhere on the rig is irrelevent.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL