Hi Walter, I too suggest you get the CC Europe volume 1 site guide. Unfortunately the new issue is not published until early january, so you will probably have to manage on 2007. Usually not all that many changes.
You've got a choice of two main routes down to the south east. One is via madrid and the other via Valencia.
There is an all year site at Aranda del Duro about half way down to Madrid, but I don't know any more down that way (there are some but I don't use that route)
The valencia route probably involves less heavy traffic - madrid can be pretty busy I'm told and probably easiest on a Sunday morning. There is an all year site at Mendigorria just south of Pamplona, although you may find the facilities severely lilmited in January. Another at Savinan, in the book under Calatayud and then two over nearer Valencia - Manzanerra and Navajas. For the first, leave the autovia for sirrion.
It can be very cold across that part of Spain in January, so be prepared and have some water inside the van. Filling stations on the autovias are sometimes a long way apart so keep the tank topped up. Manzanerra site is run from a filling station, but with limited opening hours.
From either of these you can get to the Murcia region easily in a day - sites at Fortuna or Mazzaron, and then an easy day to Almeria. You will find the HGV traffic on the autovia quite heavy round Valencia but gets a bit less as you go south.
It's about 550 miles from Mendigorria to Mojacar area where I go and then another hour to Almeria.
I should also get the Michelin Spain / Portugal map as up to date as possible. Spanish roads (autovia and major ones at least) number their junctions by kilometers so you can easliy see how far you have to go. A good system - shame not more countries do it.
Dont forget you need two triangles and dayglo vests in the car. Finally, if you are running on diesel, don't be tempted by the much cheaper gazole you may see at some stations - it's almost heating oil. I expect proper diesel to be round about 1 euro / litre in January.
Good luck and enjoy the weather, food, country and people - we have for the last 7 years and can hardly wait for January 6th 2008.