There was no text in your posting so I'm guessing your exact requirements. However, the title gives some clue. First get a copy of CC Handbook europe 1 (France and Spain) which will tell you about driving etc. and show which sites are open all year. They are mainly on the east and south coasts of Spain but there are enough inland to enable overnight or longer on way down and back. Many can be quite busy at the time you mention but are very rarely completely full. There is usually a doscount scale e.g. Up to 3 nights (top price0 then discount for up to 10, up to 12, 30, 60 etc. Some sites take the Dutch ASCI camping discount card and a few take camping cheques. For budget allow say 20 euro / night including electricity for short stay and perhaps around 15 for slightly longer. You will not get to maximum discount in 4 weeks.
In winter suggest only two entry points from France - at Irun on the west coast or north of Barcelona - either is easy and on 4 lane roads (but toll). You could consider the long haul ferries but these tend to get well booked around christmas.
Spanish main roads are generally good, but don't have rest or service areas very frequently. Fuel is a little more expensive at these than normal garages - supermarkets do not sell fuel.
Study the road signs before you go and obey the rules - on-the spot fines for relatively small offences can be high 100 euros +. But roads are not all that busy away from big cities and driving much more enjoyable than in UK.
In the coastal areas I've mentioned many supermarkets will have english foods and campsite staff some english. Get a camping card - issued free with some travel insurances - to leave in the office when checking in. You are supposed to have your passport with you at all times.Most places take UK credit and debit cards, mobile phones work well, and you can find free internet at some municipal buildings, with paid-for services at the larger campsites..
If you plan ahead and take it easy you should have no problems, except that four weeks will go all too quickly.
EHU's on site usually not above 6 amps. and may be continental sockets - take adaptor.
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