I think you better re-think either your destination, or your time frame. If you could do it in the 3 weeks, camping all/most of the way, you sure won't be seeing much as you'll be too busy riding to make it home within those 3 weeks!
Besides .... you're missing a hell of a lot of good riding areas to get to your destination. (Black Hills/Yellowstone/Beartooth/Chief Joseph/Lolo Pass/ Glacier NP/Banff NP//Jasper NP to name but just a few)
No one is completely useless ....... they can always be used as a horrible example!