I would figure at nice pace, from the Minn. border on I-90 to Rapid City would take approx. 7 hours. I used to go visit friends at college in Vermillion, SD and that trip w/o rv would take me 5.5 hours from Rapid City. Hope you enjoy your trip to the Black Hills.
Here are a few links to campgrounds in the Deadwood area,
www.whistlergulch.com
www.steelwheelcampground.com
www.hiddenvalleydeadwood.com
Here are a few more around the Rapid City area, Deadwood is only 45 mins away
http://www.rushmoreshadows.com/
www.rafterj.com
www.blackhillscampresort.com
www.theblackhills.com/mtview