There are no longer any campgrounds on the Island of Montreal to my knowledge. There are a few on the South Shore, several just west of the Island (Hudson, Vaudreuil, etc) and at least 1 nice one in Laval (North but adjacent to the Island of Montreal).
Crossing into Canada is not a problem but have your passports with you if you wish to re-enter the USA later!!! NO HAND GUNS on board or on your person regardless of any permits you may have for them in USA. Long guns (hunting) require prior authorization, I believe. Most foods are acceptable but check and answer honestly if asked about particular products. Nothing with earth on the roots for sure... Be nice and they will be nice 99% of the time.
No RIGHT turns on RED on the Island of Montreal, but almost everywhere else in the Province it is acceptable. We measure speed in Km/Hour across Canada: 100Km/h = about 62 MPH A GPS is a good way to check that as they can be set to read either kilometres or miles very easily. Distance signs are all in Kilometres.
On Edit, I should add, roadwork in and around Montreal is crazy and includes some of the bridges to and from our island, you do NOT want to use the RV for transport and parking in the Big City...
Have a wonderful vacation and Welcome to Canada and Montreal.