Manitoulin Island multi day tour?
 

Check this link. I think you may find some useful info:
Ontario by Bike - Manitoulin Island

I dream of riding there one day.

I've only been there once in my life, so no specific suggestions. If you stay at a family run type hotel one night, there's always the possibility of being able to leave the car in an out of the way place if the folks are friendly and have room.
In small towns, one could also make friendly enquiries at a local OPP station, Ontario Provincial Police.
All these depend on luck and how you approach people, but if they know you are spending time and money in their communities, chances are most likely good.
good luck

Maybe leave at the ferry terminal. It's not a huge lot, maybe room for 22 cars not using the ferry.

It's been about 12 years now since I was on a solo motorcycle tour that passed through Manitoulin island. I was on the mainland, on the Bruce peninsula, waiting for the ferry from Tobermory to the island. I was riding a BMW K75RT, laden down with camping gear.
https://cimg5.ibsrv.net/gimg/bikefor...0e7106d2b8.jpg

The ferry to the island docked, and the bow of the ferry flipped upwards somehow to show the vehicle compartment full of RVs, cars, and motorcycles. And the first two folks off the ship were these gents:

https://cimg5.ibsrv.net/gimg/bikefor...ccece6eab3.jpg

Before that moment, I hadn't realized that bicycle touring was a thing that one could do. I don't know who they are, or what their larger plans may have been. From the size of their panniers, and from the amount lashed to the rear rack, I'd say they were in the middle of a long-haul adventure. They seemed so matter-of-fact about it. When I saw them, I thought, "That looks like fun. It looks like it's more fun than what I'm doing now. And it looks like something I could keep doing for decades."

When I got back, I started riding my bike. In the space of a few years I went from sucking wind during an 8 mile commute on flat ground to 300+ miles along the GAP/C&O trails in a little over a year. So riding on Manitoulin Island has a special place in my memory -- even if I did it ages ago, on a motorcycle.

I did a short tour back in the early 90s. The island had a less developed tourist market. My prime memory is aggressive dogs snapping at my heels.
But Riding the Bruce to Tobermory was stunning

hi there J, vaguely recall you mentoring the motorcycle thing, nice Beemer btw. Back in the day, 80s, when I was still in my bike (motor) phase, I always dreamt of getting a BMW enduro and doing the whole silver aluminum case setup and riding through central America. Uni happened and then other passions and work, and the bike (motor) thing lost my interest, eventually replaced by trying out cycle touring and getting hooked on that.
Who knows, one day could see getting a bike again, hell i keep paying every year to keep my M license, but certainly hope to keep pedalling as long as i can. That's our motto, keep moving. (And why I winter commute, just to keep the legs moving)
cheers