When arriving near Montreal from Toronto, you wont have to go accross town.
You will exit towards highway30 that will take you around Montreal without having to go true it.
You can go as far as Jacques Cartier bridge, next to the bridge on the south shore of Montreal there is a marina that also have facilities for campers for 1-2 night stay with electricity and water, and also a dump site near exit. Not expensive at all.From ther you can use your vehicule and go anywear you want in town, arund 10 minues ride.Or if you wish there is a river shuttle, that will take you accross the St-Laurence river to the old Montreal, thats where visitors like to go. If you like to walk a little there is a lot to see right from there.
You should enjoy this part of Montreal.
From your camping spots, if you happen to be there at the right time you will be able to see the fireworks.
Here is the web site to the marina
Hope this help a little, and if i can be of any help, let me know.
When are you planning on being in Montreal?
Have a good trip