Sites in Bilbao

I would suggest Playa Arenillas but don’t ,know whether that is one you have tried already. 

We recently stayed on a site between Bilbao and Santander if that would do.

It's in the Acsi book 2358. The site is called Camping Playa del Regaton at Laredo. It's very popular with those using the Bilbao ferry both the night before and upon arrival. They certainly could accommodate your caravan. Might be a good idea to pre book a pitch as it does get busy when the ferry is due. We had a good night there in Sept, the site has a restaurant and the staff in reception speak English. 

Its 50 miles from Bilbao (27 from Santander) but Camping Playa Joyel, near Noja, is a big site by the beach with full facilities, shop, restaurant etc. we have stayed there after jumping off the ferry quite a number of times with  7,54m caravan (plus 5.4m pick up truck.). Plenty of room if you avoid the smaller pitches. Easy access a short distance from the motorway. 

Within the city the youth hostel at Kobetamendi  has 80 pitches which are primarily for motorhomes. I don’t know the place personally  but some reports have said they will accept caravans at quiet times. Ask the manager there.

Thank you all for your advice, which is worth more to us than Google searches. We’ll follow up with our intention to book early. Once established we can look at our next steps, being less constrained by a specific location helps.