Go for it. Look at other threads about routes etc. and don't try to get there too fast. Depending on crossing and ultimate destination, the route down the west coast of France and then over to Valencia misses potential trouble spots around Barcelona and Madrid.
Take more than one credit card and some photo ID. Dont keep all cards in one place.
Join Caravan Club or similar and get their European guides - much more informative than most of the commercial ones. Red Pennant insurance and emergency service is very good too.
As ever when travelling take less clothes and more money than you first think. In most of the usual snowbird places there are plenty of stores with british foods etc so you don't need to stock up too much.
Be prepared for smaller pitches than you may be used to in UK. If you have a large van I would fit a mover before going. Electicity may not be more than 6 amp so forget the domestic kettle and toaster. If. like me, you still use old fashioned camera film, it may be worth buying some on-line as it seems quite expensive in Spain.
Many pitches will be hard, so have rock pegs rather than plastic if you have an awning, plus a big hammer. Gas for 6 months will probably mean getting a cylinder there (Repsol or similar) Have enough on board for a week or so and then ask other "residents" - I'm sure you will soon be sorted out, although this needs a bit of thought if you van is new with the fixed regulator - you may need to arrange a bypass.
Get uptodate maps - Michelin for Spain and Portugal is good. Exits are known by their kilometers from a large city, so it makes it easy to work out how far you have to go (since mainroads are marked every kilometer).
I'm sorry if this is a bit of a ramble, but I'm envious I can't go in October too and have to wait until next january.
Enjoy.
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