Andy, we only did 3 weeks, but we managed 4500 miles and did Oregon, California, Nevada, Arizona, Colorado, Utah, Idaho. It was in the summer and record temperatures, at times it was unbearably hot and uncomfortable, but fantastic. The roads and the people are superb.
I went back again with my son but did it in a car as I wasn't sure if it would be too uncomfortable for him on the back of a bike for 3 weeks. We camped in Death Valley (not many people can say that), while there some bloke came through on a BMW and he was in full black racing leathers, I've never seen anyone sweat so much! We camped most places, Yosemite, Monument Valley, Death Valley, Yellowstone etc. If I have the money I'm hoping to go back on bikes in a couple of years, and maybe go up into Canada as well.