This website may help....
It would seem to be true that there are some restrictions, however most seem to be guided towards the trailer and not so much on the weight of the RV, with a couple of exceptions. I do know that when we passed through, coming into Canada and leaving out, the only thing they were concerned with was proper ID (Driver's license, Passport, Marriage license, ect..) The question of the weight of my RV or the vehicle I was towing behind was never brought up. Also, just a note for those that may not know, if you have a DUI conviction on your record at all, you will not be allowed to enter into Canada...again, unless that has changed.
http://www.rvda.ca/regulations.asp