There is a better option than any of the previously mentioned RV parks and it is way closer as the crow flies. These options make it easy to leave the RV behind while you are on a Cruise. There is a ferry from downtown Seattle to Bainbridge Island. On the island is nice very nice RV park with hookups. Just off the island are 3 more nice RV parks with hookup options. The ferry ride itself is very beautiful and popular with tourist for that reason. One of the big cruise ship terminals is a few blocks from the ferry. The pier 90 terminal in Magnolia would require a short taxi ride as it is further away to the NW. So if you wanted to you could leave your tow vehicle with the trailer instead of paying parking fees on it at the cruise ship terminal.
For all of you who recommend RV parks to people wanting to visit Seattle, these four RV parks are actually physically the closest ones to downtown Seattle and it avoids all that horrendous traffic on I-5, I-90 and I-405.
One is at the Clearwater Casino $30.00 a night including all utilities and free shuttle to the ferry. They have a 7 day limit. http://www.clearwatercasino.com/clea...esort-rv-park/
Just a mile or so further than the Casino is another RV park, Eagle Tree, there is free shuttle from it to the Casino but it is also on the public bus line that goes to the ferry. http://eagletreerv.com/
Also close by is the Cedar Glen RV park, also free shuttle to the Casino and on the public bus line to the ferry. http://www.cedarglenmhp.com/rv-park-1.html
Next option is a waterfront campground operated by the City of Bainbridge Island that has beachfront RV sites. It is also on the bus line but the bus only runs for commuter hours mornings and afternoons. It is the closest RV park to Seattle, just a couple of miles away and you can actually see Seattle from this park. https://biparks.org/fay-bainbridge-park-campground/
Of course all of these RV parks also have Taxi and Uber or Lyft service available for transportation to the ferry. There are always taxis waiting for passengers at the ferry terminals. The Seattle ferry terminal is of course linked to public transportation to pretty much everywhere in the Seattle Metro area. Plus there are bike rentals in Seattle and bikes sitting ready to rent right at the ferry terminal. Some of them are even electric bikes which helps with the hills. We have three companies for that Spin, OFO and Lime Bike (the one with electric bike option). It is a $1.00 per half hour ride. You need to use an app but the apps are free.
These options avoid having to take an RV into downtown Seattle as you can drive up the other side of Puget Sound using highway 3 as the primary road to get to the RV parks. (Or of course you can take the FGRV on the ferry from Seattle to get there).