Where to sleep while touring southern Ontario?
There's an app called Wiki camps which is great for finding campgrounds and works offline. 

For free places to camp Community fields can be good. I met a couple guys yesterday who said they have had good luck with firestations, they've not been told no yet when they ask. 

There’s also a free camping website which is geared towards RV parking but does have some tent listings. If you google free camping you'll find it. 

Lastly Ontario does allow you to camp on Crown land though I'm not sure in that part of Ontario there would be any to be found.