If you are planning to do a lot of driving in North America, I would seriously consider renting a car for 30 days at a time (the max. allowed by most car rental companies) on an unlimited mileage rate plan. With the amount of driving you are talking about, you will definitely make out ahead.

Just be careful of the fine print. Some car rental companies prohibit any driving outside the state, adjacent states, etc. in an effort to protect themselves against high mileage rentals.

Also, if you don't return to the renting location, you could get hit with a high one-way charge. Most car rental companies reposition a portion of their fleet from Florida to other states towards the end of Spring, and will offer some extraordinary deals to renters willing to drive these cars up North. This is another option you should consider.