About six years ago when taking the MH north after we bought it, I took 97 gal in a 100 gal tank one night - the tab was over $500US.
Two things:
1. Use Gas Buddy just like you would in the States. Different Provinces have different tax rates just like different States do, so figure out if you want to fill up before or after you cross into another Province.
2. Again plan your fill-ups to locations / towns that are just a little bit larger. If they only have one gas station then they'll get you for what they can. But, if the place has two or three streets, then go back a street or two off of the highway, and those stations are always cheaper.
Bill