Are you planning on stopping and staying at each place for a bit? How far do you want to drive in a day? What kind of parks do you like to stay in? We're partial to the provincial parks so I'll go with that for now, you may find it easier in some places like Toronto to park at a Walmart and go from there as there are no close campgrounds.
Either way, from Niagara Falls you take the QEW highway north to Toronto. The nearest campgrounds are Bronte Creek (west) or Darlington (east). Either one will require an hour+ drive with the toad to get into the city. Form Toronto you take the 401 to the 416 to get to Ottawa. You could take highway 7 as well. Along the way you have Presquile and Sand Banks parks. Both very nice, but only a couple of hours drive from TO. Check out www.ontarioparks.com for details and reservations.
In Ottawa the best bet closest to the city is the Ottawa Municipal Campground (ottawacampground.ca) Just west of the city in Kanata but an easy 15 min drive along the river to downtown.
From Ottawa it's hwy 417 east to Montreal. We've only stayed in the south KOA once. Nothing fancy but a good place to overnight and use as a base.
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