Secure parking near Bilbao for a motorhome

Is it possible/acceptable to park on a site but not stay in the van?

I can’t help on that, however our insurance wouldn’t automatically cover it. We would have to get approval from our insurer. The same applied to our caravan (different insurer). When it was left on a seasonal pitch, we had to pay a supplementary charge.

Steve is right about the need to check insurance, however overseas campsites will often allow an unoccupied vehicle to stay on a pitch at reduced rates, especially when the site is not busy. The French call it Garage Mort but I don't know if there is a comparable term in Spanish. If you want to follow it up then contact campsites in the area and ask.