In Canada(at least B.C. and Alberta) there will be a registration form. There is a portion that is torn off and given to the purchaser as proof of ownership. That with a bill of sale should be all you need at the border. You would have to get a trip permit and insurance.
I am currently looking at motorhomes in the U.S. so I have checked with both border offices. If it was registered in the U.S. the U.S. customs want to review the title and you will need an export number. I am not sure how that would be with it being Canadian registered but I would check with them to be sure.
