Washington State ferries have become expensive. An RV or a trailer are even more expensive. Ferry lines in the summer are long. We enjoy just walking on so we don't have to deal with the fery lines and it saves money. The problem is you are limited to how far you can walk once you leave a ferry on one of the islands and each island is beautiful and walk-ons can't see the islands by car. One option: we walked-on in Anacortes, got off at Friday Harbor (San Juan Island), walked up the hill through downtown to the car rental and rented one for the day for like $20 plus gas. When we were done we returned the car, the guy behind the counter drove us back down to the ferry landing, and we walked onto the next ferry (unlike all of those in cars who had to sit through 1 or more ferry landings before getting on the boat).
If you do the above, 3 state parks to stay at are Deception SP, Ft. Ebey SP, and Ft. Casey SP, all on Whidbey Island south of Anacortes. Probably need to book sites well in advance for summer months. Good luck. The San Juans are beautiful, especially if you get good weather in the summer months.
2011 Chevy 2500 Duramax diesel
2019 Timber Ridge 24RLS (Outdoors RV)
Go Cougs!