Hello Tipper,
Having spent the first 18 years of my life in SD, I know a couple of things you can do there, if you want to stop along the way. If you take I90 to Hwy 25, you can stop in world famous Wall Drug, sure you have seen the signs. On the same route, in Mitchell, you can see the world's only Corn Palace.
I hear Memorial Park Campng Area in Watertown is nice. If you have internet access, there are a few campgrounds along both directions. I just google mapped camgrounds along Hwy 14, a few state parks and campgrounds came up.
If you take Hwy 14 to Hwy 212, that will take you through Pierre. That goes over the Missouri and near the Oahe Dam. You can still stop at Wall Drug, as it is the junction for Hwy 14.Good luck and enjoy your trip.
Shane