Can you camp in Europe in the winter time? Absolutely! Just as there are "snowbirds" who head south to winter in the USA, one finds the same thing in Europe. LOTS of bargains about as well for long term campers who settle in when there are no crowds.
If you are hardy, you could also camp in Barcelona or Southern France as some do, but just be prepared for it to be a little colder and usually more rain ( maybe snow) in the the winter. In my opinion, one is better off in the deep coatal south during the coldest months of December, January and February.