I’m pretty certain it’s ABS, although a company that makes replacement f.ront panels use grp. In another Forum a company posted that rather than replace panels and disturb a lot they have a repair technique that works out much cheaper and stands the test of time.
Here’s a copy of the post. But before dashing down any line you need to diagnose the source and cause of the leak.
from 2006 to 2009 we must have replaced something like 200-300 front and rear panels on bailey, both warranty and accident damaged. since 2009 we no longerreplace bailey panels, which are merely cracked, we repair them. we consulted with bailey in the repair of the panels rather than replace and they are very happy with what we are doing and also because our body shop techs have been trained by the panel manufacturers in the repairing of them. we find it better to repairthan replace because it is quicker (ie no more water getting in), we dont have to wait for new panels and decals/badges, we are not disturbing other body seals unnecessarily and if the repair is done correctly, the crack should not re-occur.