Im traveling across Canada, want to buy campervan wont have an address. How do I register it?

Get a PO box, you cant drive it until you register it anyways

You don't. You have to have an address for registration, driver licence, and don't forget insurance. You have to be able to receive mail somewhere.
New standards do not allow for a box number to appear on a driver licence, except for certain exempt persons such as police officer, jail guard, prosecutor, etc, and registration and insurance are always based on a physical home address.