OP, my suggestion would be to plan for 3 weeks and try to find some spare time to stretch into 4 if possible. When I was a youngster we took a family 6 week cross country vacation...3 weeks out, 3 weeks back. The rush didn't afford us much time to sit back and relax and enjoy things.
Giving yourself another week (or 2) would allow you make it more of an enjoyable vacation as opposed to just a really long road trip. Day after day after day of driving got very boring to the kids... I still don't know how my dad handled driving it all!
Sounds like you will be in the Harper's Ferry area of West BG Virginia. Beautiful country out there, right near Washington County, MD and Loudon County, VA.
