If you are starting in Montana going to Niagara Falls, here are a few suggestions:
-Writing on the Stone Provincial Park in Alberta near the Montana Border
-Grasslands National Park in Saskatchewan near the Montana Border
-Turtle Mountain Provincial Park in Manatoba near the North Dakota Border
-drive the Trans Canada Highway around lake Superior
-Sleeping Giant Provincial Park in Ontario at the top of Superior
-Lake Superior Provincial Park in Ontario near the bottom of Superior (one of the best parks in Ontario)
-Killbear Provincial Park in Ontario on the shore of Georgian Bay
Your timeframe is a good time to avoid Mosquitos too. Just watch August 7 is a long weekend in Ontario and some other provinces as is Labor Day too. Parks will be quite busy on those weekends especially within 4 hours of Toronto. For Niagara Falls, there is a KOA right in town (on the Candian side), it is not cheap but you are paying for location. They have a bus that stops right outside the park to take you right down to the falls. Book early especially in Ontario, Ontario Provincial parks can start booking 5 months in advance. Also many provincial and national parks have a limited number of spots for large Class A's.