Leaking window seal - best fix that actually lasts?

One of the windows on our static has developed a leak - water is coming in along the top edge during heavy rain. I've tried silicone sealant twice now and it keeps failing after a few months. Is there a better product or technique I should be using? Getting a bit fed up of mopping up after every storm!

Silicone is the wrong product for this - it doesn't bond well to caravan panels and will fail just like you're experiencing. You want a proper caravan-specific sealant like Soudal RV61 or Sikaflex 512. The key is preparation - you need to clean the area thoroughly, remove ALL old sealant, make sure it's bone dry, and then apply the new sealant. It's an annoying job but do it right once and it'll last for years.