We have a single friend who does a Toad and manages just fine with hooking up and driving by herself.
We did one trip renting and the hassle was enough for us to Toad Up! For one trip, might well be better to rent BUT be careful that you can get a car where you are going to be. Many smaller towns do not have rental cars available or have no one that delivers. Maybe Greyline tours or the like can get you to the sights from Rapid City??
If and when you decide to get a toad, be careful as not all can be towed four down and tow dollies have their own set of issues. Dolly/hitch/wiring or baseplate/tow bar/wiring can all be quite pricey.
Driving the Black Hills/Mt Rushmore area is not difficult. Some hills and climbs present but nothing to worry about. (NOTE: There is one loop in the Custer State park that is amazing to drive but ins NOT for RVs - one way, very tight tunnel. This park is a MUST see as far as I am concerned. )
This area is one of my all time favorites. You will enjoy. BE sure to give yourself enough time to really enjoy it. Happy travels!