SD route 87 south of 16A

Your description may be a bit confusing to some, since there are several different sections where 87 and 16A run close by. But I assume you refer to the section of 87 exiting Custer SP to the south toward hwy 385 and Hot Springs SD.
I don't recall seeing any official restrictions. This section is a narrow 2 lane road, with no shoulder in most places, lots of twists & turns, lots of up and down. But its a beautiful, scenic drive as long as you can handle the sharp curves. There are official campgrounds along this section, so its passable by RV's of normal sizes.
The northern section of 87 (Needles Highway) is the one with 2 tunnels with only about 12 ft vertical clearance.

SD 87 south of 16a to 385 is passable with any vehicle or combination. There are some narrow bridges and u turn type curves, but not sharp switchbacks, shoulders and passing lanes are few. No tunnels. Enjoy the scenery and wildlife, its spectacular!

I have pulled my 5er from 16a to Wind Cave NP.

Very good! Thanks so much for the info.