Yes that will work. Don't need cell service, just connect via WiFi at home and down load the maps. You will not get live traffic if you do it this way, but no big deal.
There is a map program you can buy called Sygic where you can download whole states at a time. I think that will work better for you because Google Maps downloads are a bit limited.
If you go this route, consider a rain resistant phone. It won't save you in a down pour but will give you time to pull over and store it.