Not sure what your camping plans are, or when you're going, but if you are going to unhook and drive around I would recommend you follow 385/18 north, then stay on 79. It's a lightly traveled 4 lane road, nice and flat. Just as you come into Rapid City, bear right on 16 (it's a kind of bypass) and it will take you to interstate 90. There are several campgrounds north of Rapid and you will avoid pulling the camper up and down a lot of hills and traffic. Even if your destination is a campground in the Black Hills, ( and there are some beautiful ones) you might be better off coming in from the north. Just a suggestion. I grew up there and go back usually a couple times a year. Happy camping!
Boonieman