Hi Andrew, glad you enjoyed France. My wife and I like France, too. However, having been with our caravan to Spain three years running we much prefer Spain: more of an adventure,truly awesome scenery, and of course better weather [usually]! I don't know what route south you are taking but in January last year [2005] we travelled from Roscoff [Brittany] to the Spanish border near Biarritz in about two days. From there it depends where in Spain you want to go, and what time of year.
If you are headed due south we would strongly recommend as overnight stops the sites at Haro [halfway between Vitoria and Burgos], Aranda de Duero [halfway between Burgos and Madrid] and Aranjuez [just south of Madrid]. They are open all year, have good facilities and friendly staff. All these are in the Caravan Club site guide. From the French border to southern Spain takes about two-three days of steady driving. The roads are good and the scenery fantastic, especially travelling over the Sierra Guadarrama north of Madrid. Once you are in southernmost Spain the choice of good caravan sites is wide. For peace and quiet, excellent beaches and amazing desert scenery try Las Negras in the Cabo de Gata national park east of Almeria.
If you are staying up in the north we recommend the sites at Comillas, just west of Santander [run by a friendly and efficient Dutch guy] and Zarautz [halfway between Bilbao and San Sebastian]. Both sites are on the coast and have fantastic beach and surfing opportunities, if that is what you like. San Sebastian is well worth a visit for the day, too. The site at Zarautz overlooks a wonderful surfing beach, which you can walk to, and has a good regional restaurant on-site.
Hope this helps. John.