Hi Robert.
These sound like stress cracks in the GRP ( Fibreglass).
It's not unusual in a van over 4 to 5 years old. in fact it can occur earlier in some cases.
Advice: Only drill holes at the end of the crack if you intend to go further with the repair work ie; don't just drill holes and leave them.
But then you're getting into the realms of filling, levelling, preparing and respraying. I can give tips on this but my honest advice would be to only get involved if you are competent at auto-diy.
If you still want to drill the panel - to stop the spread of the cracks - a simple way to stop water ingress would be to fill the holes with Vaseline. A tip on this would be as follows; drill your holes at the end of the crack using a 4mm max bit at slow speed. Then take an 8mm bit and, by hand, carefully countersink the hole.Under no circumstance should you use an electric drill to countersink the hole as you need maximum control.
The countersink enables you to achhieve a much neater job.
You could also consider filling the hole and c/sink with white silcone sealer and smoothing over with a filler spreader to gain a flush finish.
Hope this helps,regards Dave.