Definitely recommend the KOA at Devils Tower - you can't beat the view! You didn't mention it, but if you wanted to visit the Badlands (east of Rapid City) the KOA at Interior, SD is excellent. (And shady! Something you won't get at the NPS no-hookup CG there.)
In the Black Hills, we enjoyed the Rafter J Bar Ranch CG. Depending on the size of your rig, I'd suggest trying for the Ranch Camp or Island sections.
Also agree with Jim Shoe about the Hardin MT KOA. It's worth spending a couple nights, if you have the time, and spend the day touring the nearby Little Bighorn National Battlefield. Nice folks running it, and definitely quiet, yet still convenient to I-90.
Billings KOA is pricey but nice; it's the "flagship" of the KOA brand. We've stayed there a couple times.
2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375