I am so sad - we were supposed to be there this week and had to cancel a month ago!
We also stayed at Custer's Gulch - nice place with hookups, but be sure you are filtered. Our buddies RV started producing brown ice after a few days and I looked at my filter and found out why! Custer's Gulch is a private campground at the south entrance to Custer State Park - beautiful park with several electric only campgrounds, lakes, and woods. Our reservations this year was within the park rather than the private CG.
As stated, check Needles highway, Deadwood, Custer State Park, and the heads. We saw tons of wildlife on the Needles highway, Deer, Mountain Goats, Rams, no bear. I didn't bother with Crazy Horse as they seem more interested in building up the facilities than they do the job on the mountain. It looks the same as when I first saw it when I was 10.
We also stopped at the oldest Nebraska state campground on the way (Chadron State Park). I think it was just electric but the campground was pretty cool - backed up to the black hills and was more of a 1930s resort than a state park. Just a bit west of that is Ft Robinson, another Nebraska State Park we visited that also has camping and is the site of an early Calvary post that is mostly restored.
Here is a pic of our last rig in Custer Gulch CG

and a pic of one of the lakes in the state park

And a pic of why you want to take your toad on the Needles highway :) That's my old dually with the mirrors folded in.
