When you first arrive in SJPP (or wherever), go to any little market or hardware store and pick up a bar of Fels Naptha. It is laundry soap in a bar and is especially made for handwashing.
http://doitbest.com/Laundry+soap+and+sp ... 632902.dib
It is very inexpensive ($1.25 in the US) and lightweight (every OUNCE counts!). You can cut it into many slices and share the cost with fellow pilgrims. Don't carry the whole thing, too heavy. It's about twice the size of a bar of bath soap. I cut a one-inch slice and that took me half way through the Camino.
To wash with it, you get your clothes wet, then rub it all over the clothes. Handwash as usual, then rinse. It rinses much better than powder detergent and the clothes really come clean.
You aren't going to find an automatic washer very often, and when you do they are expensive, so if you're on a budget, buy the Fels Naptha and you'll be set!