Though it looks like the online purchase is limited to straight forward cases. I think the length and height of an RV will require purchase at the toll booth.
https://www.wsdot.wa.gov/ferries/wave2go/For a car transaction time at the toll booth is minimal - just tell your age (if senior) and hand 'em your credit card. It's all the cars ahead of you in line that take so long!
Buying online is different from making a reservation online.
https://secureapps.wsdot.wa.gov/Ferries/Reservations/Vehicle/default.aspx