Hello there.
I am an ex-pom who has travelled extensivly across USA. My suggestion would be to plan a bit of a zig-zag between traditional Northern and Southern routes.
Including perhaps: Niagara Falls/Washington DC/Shanandoah Valley/Carolinas (my home)/Tennessee Mountains/Kentucky/Mid-West/Great Plains/Black Hills (awesome!)/Mt Rushmore/Yellowstone/ Salt Lake/Grand Canyon/Vegas.
You would miss New England and the Deep South - but could you plan another trip?
Great travels!
Jon