I've been lucky in finding commercial dryers when I've needed them to get rid of bedbugs - twice in Leon, one in town and a few years later I needed one as I left Leon and found one on the outskirts attached to a hypermarket (E Leclerc). I also needed one and found it in Mealhada on the Portugues, and another time I needed and found one in Ourense on the Via de la Plata.
I just had a look on Google maps and see there are self-service laundries (with large commercial dryers) in all the larger towns along the
Camino Frances - Pamplona, Logrono, Burgos, Leon, Astorga. Ponferrada, and Santiago. There may be others in smaller places - for example, looking at the meseta I zoomed in on google maps and found one (to my surprise) in Sahagún. I suppose it stands to reason that the locals will need a lavendería automática from time to time.
I also endorse the black plastic bag method, but in my case I've not needed it.