Took our Casita
on Washington State Ferries many times. That happens when you live on an island. You will fit on any WSF boat. They accommodate semi's.
You will be charged for length and height which works out to be about 4 times the single vehicle rate. Prices vary depending on the route and time of yesr. Some routes recommend reservations.
And yes there are propane
restrictions but you just need to turn the propane detector off at the tank.
Alaska and BC ferries also accommodate larger rigs, I have taken a 24' class to Vancouver Island and to Alaska.
Hope this gives you some comfort. Enjoy your trip.
2004 Georgetown 326DS