Related to your planning, but not for your destination, about which I know nil, be aware that the annual Sturgis bike rally is in early August. It would materially affect your trip out and back in that I-90 will be very heavily traveled in each direction from Sturgis and all fuel plazas, restaurants, rest areas, motels, and campgrounds will be bursting at the seams for hundreds of miles in each direction during the week before and after the Rally. This is not a grind against riders--some of my best friends ride, but if you're anticipating one thing on a road trip and come to find out it'll be something entirely different, it can make you change your plans. In short, I did an NC to western MT road trip in 2002 which had us arriving to the MT/WY/SD border area on the Friday before the Rally started and passing back by on the Sunday it ended. I won't do that again.
Foy