We went up to Rushmore on our 32' H/R gas V10, wasnt that bad, but it wasnt easy either.
We stayed at a C/G on the south side of Rapid City, asked the man there about going up, said it was pretty steep hill, bout 6 or 7% grade he thought.
Coming back down, there was a sign like a trucker sign to use lower gears & stated 10% grade, wow!, didnt see a sign going up so didnt know.
There were those big tour buses there & it is only one road into the place, so they made it up to there ok.
If you go I-80, stop in that Iowa I-80 truck stop & check it out. Biggest truck stop I've ever seen. Even got a place you can wash your pets, place for hair cut, Dentist & 5 different eaterys.
Neil