Well the op is fairly new to our forum,and I was wondering just how long he has been towing a van,as we have very little information,now I am no expert with caravans having only pulled one up to the lake district some years ago,but with the advantage of having a HGV1 allowing me to reverse onto the pitch with no problem and the like.
At the moment he will never get snow chains on the wheels,so he either buys a smaller set and chains,carrying chains in winter in spain is required,as Bellow has said just when you fit them is the game,and his advice I think holds good if the weather go's bad get off the road with a van,for my money that run from Santander to the Costa's is 9/10 hrs mostly motorway and with the knowledge of the weather should be no problem for the op and I suppose his co driver as they are fresh off the ferry.
Would I take a caravan down to Spain in the winter,well no,but you have to give these caravan guys their due,faced with a return ferry fare of around a grand and a mountain of stuff to get to be legal pulling a van over there ,they are not deterred,all I can say is best of luck.