We did this last year, across Spain and up over the Pyrenees into France. I'm not sure if my suggestions will be that helpful if you're planning on staying in NW Spain, but:
Camping La Gorga ( http://www.campinglagorga.com/es/inicio" onclick="window.open(this.href);return false; )is do-able in a day from Santander/Bilbao. We arrived at 11.15PM having been driving flat out through the Spanish countyside for hours round twisty backroads, and were given a warm welcome. It was accessed via a narrow, rickety bridge (below).
We had a great time, and found the Spanish drivers tended to be more tolerant of a couple of slow cars than the French were!
We also stayed in a couple of campsites nearer to Pamplona, but can't find their details right now. To be honest, we found all of the campsites in Spain to be well equipped, clean and friendly - almost all had swimming pools, too - a real bonus in July. As a group of 6 of us, with a tent, camper and car, we never paid more than 55 Euros for a night, without needing to book.
I'm sure you'll have a great time