Crossing the Cascades, I recommend you not take US-2 or I-90. That puts you into the city of Seattle.
The worst traffic on I-5 is south of Seattle traveling thru Tacoma. Coming up from the south you can miss the worst traffic getting to the Kitsap Peninsula/ Bainbridge.
Taking the ferry across the sound to Bainbridge sounds exciting, but the roads to the ferry terminals in Seattle will definitely not be an RV friendly drive. And the ferry is not cheap. You will have to reserve your crossing far in advance. Unless you can arrive a few hours early, you risk missing your reservation if traffic causes more than normal delays. Frankly if you are not used to taking your RV through major city traffic, worse than Dallas, Houston or Atlanta, you want to avoid Seattle.
You can cross the sound on the Tacoma Narrows Bridge, but traffic both north and south of the bridge will be at a long stop and go pace.
The easiest way onto the peninsula will be WA Hwy 3 from US-101. Turn west at Olympia to get to that route.
US-12 across the Cascade range is s scenic drive, but some folks will not be comfortable with the up and down grades.
Full-Time 2014 - ????
“Not all who wander are lost.”
"You were supposed to turn back at the last street."
2012 Ram 2500 Mega Cab
2014 Flagstaff 832IKBS TT