The couple of time I've been to the spanish GP we've stayed at El vedado
Campsite link Vallromanes 15 or so mins from the circuit nice little bar serving food good eateries just down the road too. Just make sure you take extra care with you gear wherever you end up pitching as last time we went we got turned over by some other campers on Saturday night whilst we were sleeping.
The Circuit site is a pretty rough and ready affair unless it's been developed in the last few years, I'd give it a wide berth.