Don't shortchange yourself. The Yellowstone/Teton stay should be 6-7 days just to get the tip of the iceberg. A lot of people don't realize how HUGE Yellowstone National Park is. Grand Canyon doesn't take long to see in my opinion. I would choose one or the other. (I would definitely choose YNP/Teton). On Your way to Yellowstone you can spend a day or 2 @ Custer State Park, Badlands and Mt Rushmore in SD then head on over to Yellowstone for a few days. While you are there try to take the drive over Beartooth highway (not in RV). Then to Tetons for a couple of days. On your way home go south to Colorado to the Durango/Silverton/Ouray area and ride the train. Drive the "million dollar highway" (once again, not in RV). These stops will fill your 3 weeks very, very full. Have fun whatever you do. Enjoy!
