We too have been to Spain for a number of years, but not until immediately after Christmas. Some years ago sites could be quite full at that time but more recently this has not been the case. Recommend the CC Europe 1 guidebook which shows open all year sites.
You may find some pitches are reserved but this is usually because most sites have "regulars" who like to be on the same pitch each year and reserve them for the next year on leaving around Easter.
You will probably find that there are significant discounts for long stays with several discount bands typically after 3 10 30 60 and 90 days (with all days at the lowest price). The high charge for the first one to three days is said to discourage overnighting by "wild campers" who come just to use facilities, laundries etc. but unfortunately it also applies to thoses who are in transit and only need to stay one night. Earlier this year, the higest price i paid for one night was 26 euros whereas my over 60 nights long stay was 12.5 euros/night.
As John has said there are plenty of sites along the coasts, but not so many inland open in winter. The same is true in France if you are taking a short or medium distance ferry , not going UK to Spain directly. Living in Somerset, i use the western crossing and then down the Atlantic coast of France to Irun, then across to Valencia. Popular OAY sites in France include "Futuriste" near Poitiers, then La Rouletta at St jean de Luz - virtually on the border. From there it depends on your intended destination and how long a drive you feel comfortable with, but one or two more stops should get you most places in Spain.
if you look at some of the earlier postings on this topic you wil find lots of information and suggestions about routes and stops.