I have not taken the BC ferries, but I have taken the Washington state ferries through the San Juan islands many times. Low tides are a SERIOUS issue. If the tides are particularly low, they will NOT load any RVs at the Anacortes landing. I would certainly check tide schedules and ask when you make reservations about loading of large vehicles. In Anacortes, reservations for RVs are required, I don't know about Vancouver. Ferry scheduling of RVs requires that you be very flexible and not on a tight schedule. Good luck on your trip and enjoy Vancouver Island.
Richard