Wes,
The answer depends on many factors; is the van still under warranty? how old is it if not? do you want to do the repair yourself? and do you know if the panel is made from GRP or Acrylic capped ABS?
Having said all that, and if it's GRP - you probably have a stress crack in the Gel coat that covers the GRP. These cracks can be scratched (or ground) out and then filled with GRP. The repair will be as strong as the original.
But if it's a through crack, which is a lesser possibility in GRP - then it's more complex to repair, but still possible for DIY.
If it's a crack in Acrylic capped ABS, then that requires a specialist repair kit, and is probably outside the realm of DIY
If it's under warranty, get it professionally checked and either repaired or the whole panel replaced.
Don't use silicone bathroom sealant (or similar) as a temporary patch because that will just make the real repair more difficult...
And are these likely to let water in? Yes - a surface crack in the Gel coat will eventually cause the surface to become loose, a through crack will let water through - but it's never going to be a torrent.
Robert