I spent quite a bit of time in Northern Spain in 2012 to mid 2014. During the weekends could sometimes get a short trip in for sightseeing. Along the northern coast there were a number of RV Campgrounds, all seemed to be extremely cramped, small, and crowded during the summer months.
The month of August is essentially the month that most people take a bulk of the vacations. I know we actually had to schedule 2 weeks off per person during this month, it was a nightmare from a management standpoint but the employees enjoyed it.
They do have RV rentals if you do some searches. All of the RV's I saw were the small Class C or B type. Do not recall seeing a Class A in the ~200 days I spent there. I do not recall seeing any grey/black water dumps anywhere and unfortunately you did see where people would just dump in open lots.
Over the last couple decades the Spanish government build an amazing highway system (using loans from the Eurozone nations). The scenery along the northern coast was breathtaking, small coastal towns were quaint. Again, extremely crowded during the summer months. I did see that in some of the coastal villages, near the fishing docks, there were people boondocking. The weather was cool enough where you probably could camp without AC.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi