You will have good cell phone signals along the interstates, especially Verizon, which is much better in TN than ATT. At times it may drop to 3G. Set up your smart phone to use as a hot spot. Verify that you can also talk on it at the same time. If not, you could get a second inexpensive phone just for talking on.
One of you could work while the other drives. With 3 weeks to work with, you should just be able to plan 2-3 days at a time. IMHO that's the best way to plan a trip anyway. Make a list of your must-sees, so you don't miss any of them. Make sure you have reservations for Friday and Saturday nights.