Sounds fabulous!
You're ok to enter for upto 90 days on the esta.
We've done several road trips to various parts of states but for maximum 18 days.
For a starting point I used to get brochures with escorted tours eg Titan tours and also virgin have fly drive holidays.
We've done over various trips - california, new england, deep south, virginia, tenessee, washington DC, new york, memphis, nashville, vegas.
With that length of time i'd be tempted with coast to coast or route 66.
Once we'd decided on our area and approx route i'd get a guidebook like fodors and plan away. Unless we wanted to stay somewhere special like a b&b we just used to book accomodation each night.