Welcome to the RTA Forum!
What was your though process on heading down to Cheyenne and then across Nebraska and Iowa? It's not necessarily a bad choice, but it's not the most popular one.
Most people making this trip would leave Yellowstone to the east, either checking out Cody or going over Beartooth Pass, on their way to South Dakota. Going that way would take you through the Black Hills, with places like Devil's Tower, Mount Rushmore, Custer State Park, Jewel and Wind Caves, and then the Badlands.
Like I said, going across Nebraska and Iowa isn't necessarily a bad choice, but that's another option for you to consider as you do a bit more research and planning.