As with all big cities finding somewhere to camp within public transport distance can be a problem. In the case of Montreal the best we have found is a good KOA campsite south of the city near Saint-Philippe (N45.34750 W73.48968). From here there is no public transport but it is relatively easy to drive to a park and ride. (North about 20km along the 15 (also known as the 132) to exit 7 (not 8) and park in P5, then walk to the metro.
We found Montreal on a very hot Monday (with all the museums and galleries closed) a bit of a disappointment. The old town was OK, but not up to Quebec standards.
Some people found it all a bit too much.
.. we did find a very good restaurant for lunch, L'Usine de Spaghetti in the old town on Rue St-Paul Est.