There are a couple of campsites near Bilbao (not sure what time they accept arrivals till). After that very little for a good while. We normally just overnight at services tbh. Never felt uncomfortable.
Camping Sopelana is close by ... are you planning to stay a couple of days, or, just overnight and travel on next morning?
Which direction? East? West? South?
I might be able to suggest better if I have an idea of what you intend.
Don't forget to leave a review of the French and other European campsites you have visited!