Crack above window

I think a photo and a bit more detail might help members to come up with suggestions.

Colin

If it's possible you should stop drill the end of the crack to prevent further propagation. Also, as Colin has said above, a photo would be of help.  

And the make, model and year for the caravan.

try contacting eeco in Shelf  near Halifax they can make and repair windows I had a window repaired by them it only took a couple days for them to do the repairit might be worth a phone call before you drill any holes in the plastic

 Unit 12 Grove Mills Wade House Rd Shelf HX3 7PE       ph 01274679524 mon -fri