we just did that trip last summer. it was one of our best.
a few comments. Parking at mount rushmore is 1/2 off for seniors.
the senior pass is worth it for devils tower(free) and the badlands(free). we got our lifetime pass a few years ago when it was $20 just before the price increase, is it $80 now?
we've camped at devils tower koa a couple times and the belle fourche campground twice since at the tower. much preferred belle fourche as it saves a boatload of $ but no electricity.
didn't do caves this trip, but have done them and they are both pretty good. unless you are a gambler, skip deadwood.
we stop at wall drug kind of as a tradition every time we are out there.
great scenery and tons of buffalo in the badlands. cedar pass campsites are just bumpouts along the roads, but it is convenient and crowded but quiet. note: the shower building doesn't have interior lights so take your own or shower during daylight. one little kid was in there with his father and screamed at a level that hurt my ears as he was afraid of showering in the dark.
good food at the longhorn cafe in sundance (on the way to the tower) it is easy to find as half of a longhorn is sticking out of the front of it. unfortunately the circle b cowboys cookout/show is out of business. it was the best of all the ones we've gone to around the country.
spokane ghost town was a disappointment.
the metal store in hill city was good. amazed at all the stuff the guy makes out of metal.