We took the ferry from Vancouver to Vancouver Island and it was $177 on their Wed and Sat discount fares. On the way home we took the Victoria to Port Angeles ferry and it was $325. Your coach is about 7' longer so you would pay more. A day trip to Victoria is not worth it. You will spend 3 hours on the ferry and 6 hours in que and customs/border patrol. My suggestion is to spend lots of time on the island or skip it.
We spent 11 days on the island and needed 3 times as much, Three of those days were in Victoria and we needed a week or more.
Our trip down 101 was nice, but the campgrounds are really full this time of year. By accident we found a campground just south of Raymond, but before South Bend. I don't know the name, but it was a catamaran building site and it over looked the bay. The cost was $20/night with FHU. Not fancy, but nice and cool with great view.