With the route you are suggesting, it would be the Tsawwassen (you don't pronounce the T, so SaWaSen) to Schwartz Bay outside of Victoria. I would highly recommend that you make a reservation on the website Grassilander posted. It's only $15 and with a big motorhome will guarantee you a place on the ship. There aren't usually reduced fares, but sometimes the do Coast Savers and you would see those on the website. Sunday evening/Monday morning and Friday evening and prime sailing times, so avoid those if you can.
Back to the mainland the Nanaimo/Departure Bay to Horseshoe Bay is nice................unless you are dropping the motor home at the airport, in which case the Nanaimo/Duke Point to Tsawwassen is the best bet. You are only 25 minutes from the airport there.
As for camping.................sorry, we gave that up before we came to the Island so I can't help you out.
H & G