I have stayed at the Northern Lights
Casino near Anacortes Washington a couple of times on my way to and from someplace else and I have not able to get into a proper camp spot. They have a clean fully service RV park with ocean views for about $24 a night. Close enough to Anacortes that you can head there for a good dinner. If the RV park is full you can park in a seperate area of the parking lot for about $7 a night. Security comes by often and there are often a number of other campers using the area.
I also ended up recently spending the night at the Angle of the Winds Casion in Arlington Washington when all else failed. They have new RV section in the parking lot with power supply - no water but they have a water filling station. It was $15 for the night and they gave a $5 coupon for the casino and I think a coupon to cover a breakfast. It was fine but basically it is camping in a parking lot with power. :-)
I find casinos to be a better option than sleeping in a Walmart parking lot if you just need a place to stop for a short sleep. Have never felt unsafe as there are always lots of other campers around you.