There's several places where you can do this in the park, not just one. Some of the most popular are on Canoe, Tom Thomson and Rock Lakes.
Most of them aren't exactly near a place you can an RV into though, be prepared to canoe for some distance to get to them.
Ask around locally too, some of them are definitely not for the faint of heart. It's 30++ feet down to the water.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).