To the OP,
I'm at an RV park in Sequim.
Peak Season for the WADOT ferries is May 1 through September 30.
Mount Vernon to Sequim:
via Coupeville/Port Townsend ferry: Rt 536 to Rt 20 to Rt 101. about 82 miles. Most direct route, but may also take 5-6 hours to complete - based on ferry schedule. If the 2017 ferry rates don't change much from 2016, a 60' to 70' RV costs about $101 USD (1 way) during peak season.
via Edmonds/Kington ferry: I5 to Rt 104 to Rt 101. about 100 miles. more frequent sailings, costs more. a 60' to 70' RV costs about $131 USD (1 way) during peak season.
via I5/Tacoma: I5 S to Rt 16 to Rt 3 to Rt 104 to Rt 101. about 190 miles. no tolls.
via I5/Olympia: I5 and Rt 101: I5 S to Olympia to Rt 101. about 230 miles. no tolls.
Rest-A-While RV park is off Rt 101 in Hoodsport, WA. 70 miles south of Sequim
My thoughts:
I don't like taking an RV through Deception Pass (Rt 20). Road is twisty and narrow. If there is construction or an accident the road comes to a stop. Guard rails in places look like they were built during the WPA, or CCC, or chain-gangs, or by Fred Flintstone and Barney Rubble.
I've never taken the Kingston ferry.
Driving I5 from Joint Base Lewis-McCord to west of Olympia sucks. West of Olympia, all the way up Rt 101 to Sequim is a nice Scenic drive. Views of Puget Sound, Olympic National Forest, small tourist trap towns, yadda, yadda, yadda.
I5 to Tacoma, up Rt 16, is probably the most cost effective route to take. You'll have a view of the ships docked at Bangor Naval Base.
RV parks in the Port Townsend/Sequim/Port Angeles area:
Escapees SKP park in Chimacum, WAFort Worden in Port Townsend Diamond Point Resort in SequimSequim Bay State ParkJohn Wayne Marina in SequimGilGal Oasis RV Park in SequimRainbow's End RV Park in SequimConestoga Quarters RV Park in Port AngelesSalt Creek Recreation Area