Have you tried a scrappy for a replacement second hand skylight as you may fix this crack only to have another one appear a couple of months later due to the plastic trim becoming brittle? If gluing be careful not to bond it to the roof of the caravan.Sadly the van is now 6 years old and out of any warranty. I've been researching glues and there are a few out there designed for just this purpose, essentially liquid solvents which flow into the crack by capillary action and weld it, (weld-on 3, tensol 12 etc)
I assume it would be best to remove the light so gravity helps the liquid flow into the crack. When we can finally get the van back home from storage I'll give it a go and report back.