I agree, RT 66 is way overrated and hardly worth it.
With so many great national parks, great roads and a fantastic climate in June/July, go to the National Parks, Bryce, Moab, Rockies, Grand Canyon, Yellowstone, the California Coast, Yosemite etc.
Lookat EagleRider, each franchise in a major city is usually associated with a motorcycle dealer, and you can rent a Harley Davdison (not recommended but whatever floats your boat), BMW, or Triumph. You can rent it oneway too, and pay a drop fee, then fly back, or do a huge loop.
The advantage with EaglerRider is all the dealers, regardless of make of bike, support the program, should you have an issue, they will respond, pick up and replace the bike, good piece of mind. They also tend to be well mainatained bikes usually less than 2 years old.
I would look at getting a BMW 800GS, 1200GS, Triumph Tiger 800/XC or 1200 Tiger, with bags, and get a nice tent/waterproof duffle, GPS and hit the road with a good atlas of the states parks, camping and secondary roads. Just be prepared for cold nights in the mountains.
Or, as above say, buy a nice Japanese used bike and get some new rubber, service it and go,,,far cheaper. Sell or trade in at dealer when done.