I have never been asked by the US for how long I am going or to show any proof of my Canadian residential ties. But the could, of course. Normally, DL is the proof enough. It is an offense not to change the address on DL and on vehicle papers when you change your physical address permanently - it has to be changed within 2 weeks. You may also keep the copy of your tax papers with the same address.
It is Canadian medical plan and GIS social pension that you should worry about. Your annual absence should not exceed 7 months for medical plan, and uninterrupted absence should not exceed 6 months for GIS. Additionally - and they do check this from time to time - you have to satisfy CRA that you have "significant residential ties to Canada", check their
Circular SS-F1-C1, par. 1.10-1.17.
As you will see, it's not just a matter of renting a mailbox in Canada. Things like mailbox or safety deposit box CRA considers to be "of limited importance" in determining your residency status - see par. 1.15 in the above Circular.