Hello Nick, Perhaps I'm becomming too hard headed about this, but if you know the damage has occured because of some abnormal usage or cleaning, ok its down to you, but otherwise, why should the window fail? Even if it is out of of the stated warranty it must have been faulty workmanship or materials in the first place, and I would vigerously point this out to dealer. ask for a replacement, after all when you ordered your caravan did you state that you wanted faulty window? No, so why should you suffer the cost when it not your fault?
Cracks in acrylic windows (if they are truely acrylic and not polycarbonate) can be repaired quite well using polystyrol cement - the same as used for plastic models. Be careful to avoid excess adhesive on the visible surface though, as it will go milky and don't wipe any excess off, just let it dry and cut and polish the area afterwards. However, there is a danger of hairline cracks growing again even after repair. This can be prevented by drilling a small hole at the end of the crack (about 3mm dia. will do) and fill it in again with a transparent glass repair resin compound, like that used to repair chips in windscreens.