Washington State 210 day law, does it exist?

I don't think it is a state law. Otherwise one of you all would be citing chapter and verse from a .wa.gov link. I did find this link from the WA DOR though:
http://dor.wa.gov/content/contactus/con_residdef.aspx

The 2nd bullet of their residency definition says:

Lives in a motor home or vessel which is not permanently attached to any property if the person previously lived in this state and does not have a permanent residence in any other state;