Haven't done all of it but...
If you aren't set on sticking to the USA, I would look at doing a circle tour of North America (you could start anywhere just watch the seasons or do it over multiple years):
- Starting in Cartwright Labrador (eastern tip of mainland Canada), drop down thru Newfoundland
- Then take the ferry to Nova Scotia
- Cross into Maine and follow the coast to Key West.
- Then around the Gulf of Mexico to Rio Grande Valley in Texas.
- Depending on your sense of adventure/security situation, you could run down as far as Panama before coming back up the Pacific coast.
- Alternatively, cut across to San Diego CA and head north to Seattle/Vancouver
- Follow the Casiar Hwy until you connect with the Alcan and then follow around to the Kenai Peninsula in Alaska.
- Completing the north leg, circe north to Fairbanks and then take the Top of the World Hwy to Dawson City.
- Side trip to Tuktoyaktuk to dip your toes in the 3rd Coast (Alaska Haul road, you can't drive to the ocean but on the Dempster Hwy in Canada, you can camp right on the shore).
- Take the Klondike Hwy back down to Watson Lake.
- Loop back north a bit to see the coast of Great Slave Lake.
- Cut between the coast big lakes down into Winnipeg before cutting over to the north coast of Lake Superior.
- Follow the north edge of the Great Lakes coast to Kingston.
- Possible side trip to see the coast on Hudson Bay.
- Follow the north coast of the St. Lawrence Seaway back to Labrador.