We've done a similar trip Detroit -> Rapid City (twice, both with pull-behinds).
Once you get on I-29 to I-90 its all corn fields--very boring (but relatively easy) driving.
The terrain along I-90 in South Dakota is really long rolling hills (so long inclines followed by long declines).
The other experience we had along I-90, in the flatter part of the state to the east, was the wind: Since its so flat the wind just howls across the prairie and when you come to a bridge over the interstate it would seem to channel the wind along one side of the bridge. We got accustomed to readying ourselves for a blast of wind when we would emerge from under a bridge.
You have to make a stop at the Corn Palace in Mitchell, SD and at Wall Drug