The International Capming card is generally accepted instead of a passport and is well worth carrying.
If you get Caravan Club volume 1 you will see all the sites in Spain open all year. Some of them on the costas will be quite busy, although the credit crunch may reduce numbers a bit.
Many sites have a discount structure whereby you pay quite heavily up to 3 nights and then progressively reduce to about half price at 60 days or more, so look at the tariffs.
Electricity can be fairly low voltage and not more than 6 amps, so no point in taking the domestic kettle or toaster.
Supermarkets do not generally sell motor fuel (or pharmacy items).
Credit cards generally accepted with ID e.g. photcard driving licence.
You need one reflective jacket for each seat in the car, plus two triangles. Guarda Civil drive green and white cars and 4x4 -reputedly no unmarked cars. Europe volume 1 has lots of other information beyond sites.
There may be a problem buying spanish gas cylinders except Camping Gaz, so plan how much to take; you cannot get UK Calor refilled or exchanged.
it can be very cold in Northern Spain in January, so be prepared.
Depending on which ferry crossing you use, the route down the west coast of France and then Pamplona, Zaragoza to Valencia may be easier than central France towards Barcelona.
Road tolls in Spain are generally a bit less than in France. Autopistas - toll or not - are generally very good and well signed, although without too many service areas, so suggest keep the tank topped up.
English is widely spoken in the main holiday areas, but less so in rural areas inland.
Hope this helps and that you have a good first trip - hopefully the first of many.