On our winter trips to Spain but we have only ever travelled through France.
Suggest that you buy the Caravan Club Europe book they have lists of campsites which will be open that time of year (not many but they are there). It is in 2 volumes so make sure you get the one with france in it. A straight line on the map from Calais to Perpignan gives you a route through Rouen, Dreux, Evreux, A71 to Orleans, Clermont F. on A75. This avoid Paris and is quite cheap on tolls.
The A7 on the South Coast of Spain will take you as far as Malaga but, be warned you need to be very aware. Don't leave the van unguarded in motorway services. And travellers with big smart caravans and vans are seen as rich and open to being stopped on the motorway to relieve you of your money, phone etc.
We have never had this problem so don't let it put you off. (But we know some who have)
But we always carry a whistle, mobile phone ( we all do now, i know) and a short steel bar to hand in case someone gets in the van uninvited.
We used to sleep on French motorway service areas but wouldn't now and definitely not in Spain.
We have camped many times at Vinaros (Camping Vinaros), not the prettiest of places and not far south but it is quite warm and because it is a fishing and manufacturing town and not just a tourist town it is not all closed up in the winter as some places can be.
We avoid Barcelona, take the inland motorway otherwise it can be dire (having done it once by mistake).
A day trip driving inland from Vinaros can get you very quickly to some brilliant walking country, gorges, mountains, dams etc. And the footpaths are now much better signposted.
On the return trip we used to go inland to Lerida and then to the tunnel at Vielha which will eventually brings you out to Toulouse. Driving can be a bit hairy though.
It's a great place to go in the winter however you travel, so have a good time.
David