You would be surprised at the number of insurance policies that have limited Canada / Medico coverage. Especially if you spend significant amounts of time there (3+month). Generally people are ignorant of this unless they actually read the fine print of their actual policy which is usually found online these days. Even agents and phone reps are clueless.
Some companies will get you if you have an out of state policy in states like NJ which have unliminted PIP clauses. You can run into issues if you enter a state like that which a company doesn't have a presence for a particular reason and end up staying there for a long time meeting registration requirements which means in state insurance.
Most of the time its not an issue because people don't get into accidents that often in our rate group and people generally don't stick around long in states other than their registered one but if you have a serious accident the company might start looking for reasons to not pay.
The rules that govern insurance / vehicle registration are the same regardless if you're a full timer traveling, spending 4 months visiting mom and dad or some jack-a-mo who moved from Maine to Mass and wants to keep his Maine insurance and registration because it is much cheaper.