Spanish roaming

I have been following the discussion about roaming unbooked in France and how easy it is to find sites without pre-booking. We are planning to go down to Spain through France this winter and maybe stay there for 2-3 months. Is it as easy in Spain to do it ad hoc? We have looked at some of the CC recommended sites in the Winter Breaks Catalogue and the ones in the far south seem to be booked up in Jan & Feb. Is this usual all over or are those ones just full of CC members who like to book ahead?