Not too big of a problem.
Take the Rockguard off. Level it where the crack is located.
"Carefully" inject into the crack, a plastic solvent, such as IPS # 4, using a small syringe and needle. Use a very small amount.
Do it 3 or 4 times. Let it set up overnight.
Invert the Rockguard, and do the same thing on the opposite side.
That will fuse the plastic back together, and will look ok.
Most importantly, it will stop the crack from spreading without having to stop drill it.
Ah, such is life with an Airstream.
Always something to do.