Moisheh, The first issue to watch is where are you taking it before you register it. Some states don't recognize paper (temporary) registrations and will consider you are driving an unplated, uninsured, vehicle. One I know of like that is Massachusetts. If you get stopped it's driving without insurance, they seize the vehicle and drop you in jail until the hearing. So you need to check with the state police of every state you plan to cross to see if they accept the paper or license plate and insurance you will be carrying. Most will laugh and say you're good, but some may not.
If you leave the Ontario plates on it, Canada Customs may just figure you're driving a friend's unit home for them, but you want to have the story you're telling thought through with paper to back it up....in other words a letter from the executor or wife saying thank you for bringing our motorhome home, if that's the story.
If you plan to register it in the US, you need to take it to a US Customs Port of Entry (there's one in every state, I think) and import it. They will check the manufacturer's plates and serial numbers against the registration and check when and where it entered the US. takes maybe 45 minutes total. I used Tampa which is behind the airport both times I did mine. Of course, I was the owner and importer both times.
As far as registering it in a US state, The rules vary by state. Some states want it registered where you live, so you'd have to be legally resident to register it in that state, but some states go by where the vehicle lives. That makes it easier as arranging storage for over six months a year (or a campsite) means the vehicle lives there over half the year and must be registered in that state. For example, one rig I had was registered in North Carolina and I paid $75 for a spot in the off-season in Mt. Airy, NC so that it was legally in NC and had to be registered there. That turned out to make it eligible for personal property tax in Mt Airy County every year, but oh well.
