Just for planning purposes, Bellingham is about two hours north of Seattle, depending on traffic. If you are pulling a boat behind your motorhome, you will definitely want to check with the office at Squalicum Harbor (Bellingham) for parking info.
If you aren't bringing a boat, and are planning to bring the motorhome to Friday Harbor, you will have to put the motorhome on the ferry at Anacortes - when we brought our small motorhome with a small cargo trailer to San Juan Island (where Friday Harbor is located), the cost was around $350. There are NO full service RV sites on San Juan Island. There are about 8 RV sites with electric and water at the fairgrounds or at Lakedale Campground (about 8 miles out of Friday Harbor), a similar situation with electric and water only. No sewer connections, but they do have a dump site.
Because of the cost of bringing an RV on the ferry, most people don't... thus, the reason there are no full service commercial sites on the island.
The other option is leaving your RV in Anacortes and walking on the ferry to Friday Harbor (about $12+ per person). The nearest RV park to the Anacortes Ferry Landing would probably be Fildago Bay RV Resort (several miles out of Anacortes, then another 4 miles to the ferry landing).
The best way for a boat owner to get to Friday Harbor from the mainland is by private boat. Then, there is that issue of what to do with the motorhome while you are gone.
My wife and I have worked in Friday Harbor the last 4 summers, and have spent quite a bit of time cruising in the San Juan Islands. We are RVers, so we understand that aspect of it, too. Drop me a PM if you have any questions.
Best wishes,
Jim B.