WATCH the tide tables, your 30'er is boarder line, when the ferry is docking at low tide the ramps are very steep. you may drag or high center. they also cancel the runs at the lowest tides. The Whidbey side is too shallow to run the new ferry at the lowest tides. Mid tide or high tides no problems. and you will need a reservation. Turn off your propane. they will give you a tag for the tank.
We live on Whidbey. Have fun, Oak Harbor is a full service town with all the stores you may need.
When you get off the ferry on Whidbey, take the left hand turn to Coupville, the right will add about 10 miles to your trip north.