In Ontario, the provincial parks are pretty good for tenting spots but there are a lot of RV spots and facilities too. They often book up months in advance but sometimes they have space available during the weekdays. The more popular parks tend to book up quickly. The sites are usually way more secluded than private campgrounds but they are pretty expensive (almost 50$ a night in some spots) due to the gov't cutting most of the funding for the parks, so they have to raise the money from fees. Most parks that have car camping will have good showers and some sort of store as well but some are quite a ride from the main road and a lot of the provincial parks are just "wild" parks so check out the parks website if you are looking at possible destinations, some of the parks include significant cultural or historical sites. Some of the provincial parks have non-reservable sites, or so I was told by a worker at Pinery but I don't know how to find out which that do beyond calling the actual park and asking them.

I have had good luck staying at private campgrounds but haven't stayed at many in Ontario... one time the owner upgraded us to a cabin due to an impending thunderstorm at no extra charge. At another place that was mostly RVs the owner offered me use of her trailer to store all gear so the bears didn't get at it, a lot easier than bear-bagging.

FWIW, just in general, I would say Americans and Canadians lie somewhere in between Germans and Spanish in terms of flexibility in plans and doing things according to protocol. (I lived in Spain for a long time, and just about everywhere you could always find reservations, no matter the situation, after some polite banter. Some exceptions in Catalonia and Euskadi, but very few. The Germans I knew tended to not even consider anything outside of the specified norm, as if even trying was very impolite. Neither is good or bad, just my general observation.)

That said, and as Macha said, a lot will depend on weekend vs. weekday and the area. If you are in Yosemite or Banff on a three day weekend, you might not find a place. Also keep in mind, our tourism here in N. America is not as season specific as in Europe, (though from May-August is somewhat higher, just not the huge spike like in Europe).

Lastly, don't be afraid to ask people. Most N. Americans, contrary to rumors, love to show foreigners and out of towners the best of themselves and their homes. I assume you're on warmshowers, but if not, that's another highly helpful place for touring.