A few suggestions:
1) First half of your trip you got to be in the north side and then move south as the weather grows colder.
2) Consider a comfortable and reliable vehicle such as a BMW K1200LT or at least BMW R1150RT. These have all the comforts and reliability that Harley cannot compare on. I have done 1200 miles a day on my BMW. But I understand that you may like a Harley better. Choose your ride but BMW will be cheaper and more economical if you buy used. Makes more sense for extended travel.
3) I don't know where you wish to start and end your journey but you could go start from NY and travel across USA in anti-clockwise direction and end your trip in Miami in December. Here's how: start in New York and see Niagara falls, NY City etc. head towards Seattle seeing Chicago, Wisconsin, Mt Rushmore, Badlands National Park, Sturgis in August (must see if you are on a harley). Then quickly visit YellowStone National Park, Grand Teton National Park, Glacier National Park. Head out west to see Olympic National Park and Seattle area. By then it will be September.
4) Then head a bit south to Oregon and make it to California along the coast. Choice is between scenic Hwy 1 vs Interstate 5/Hwy-101. Very beautiful indeed. Check out San Francisco, LA and San Diego. Check out Sequoia National Park, Yosemite then come back towards North east visiting Grand Canyon, Las Vegas, Arches National Park, Zion National Park, Rocky Mountains and Smokey National Park for the fall (lovely colors in fall). Maybe you will just miss fall.

5) Then you head south. The Gulf coast near Mobile, Alabama and Florida Panhandle is quite nice actually with lots of coastal towns. New Orleans is cool too. Then as nights get colder move east and south towards Miami and go to Key West. While rest of the country freezes their balls off and shovel snow you will be thinking of your next trip to nowhere in particular.


Have fun, mate. Above all - take it easy and be safe. If you need a place to stay in San Francisco area, let me know. I may have a guest bedroom for you to rest yourself and share stories.