If you are CC members then the ferry and camping cheque booking is usually a good deal. Also their CC Europe Spain site guide volume will show you plenty of choice of sites at that time of year. Be aware though that many Spanish sites have a charging structure which starts high for 1 to 2 or 4 nights, the drops quite sharply for a longer stay and progressively lower for even longer stays. This means a series on 'overnights' can run up the total site fee quite a lot. The good news is that the autopistas and autovias are generally very good and not overcrowded unless round the major cities, so it's fairly easy to get a good distance in each day. Some sections are toll, but not as expensive as in France. Supermarkets do not gernerally have filling stations, so there is not much differnece in fuel prices although the autovias may be a little higher. On some sections it can be quite a distance between services so it's not a bad idea to fill the tank at half rather than quarter full.
On your route there are sites at Burgos, and Aranda north of Madrid. A place worth a day or so stay is Merida with Roman theatre and ampitheatre in cluding a chariot race track - unfortunately no longer in use !
Sites with chalets to let ( which seems to be most of them these days) and near the cities may attract groups of young folk at weekends who often tend to 'party' until quite late.
Hope this is helpful and that you have a great trip
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