There is no special reason for Myrtle Beach, if you say Charleston is a must I will look up some tours and attractions that might intrest us and cancel MB out.
Our tour right now looks like this:

Charlotte, NC - 1 day (arriving late, so I booked a room)
Charleston, SC - 2 days (instead of MB and 2 days Savannah)
Savannah, GA - 1 day (I take alternatives - Saint Simons Island maybe?)
Jacksonville, FL - 1 day
Panama City Beach, FL - 2 days (beachy vacation kinda thing)
Mobile, AL - 1 day
New Orleans - 3 days
- flight to Dallas - 4 days - flight to Nashville
Nashville, TN - 3 days
Chattanooga, TN - 1 day
Great Smoky Mountains - 2 days
Charlotte - 1 day - flight back

The Bulls Island Ferry Expedition Tour looks amazing!