This state has BEAUTIFUL state parks that offer huge sites and great camping. But keep in mind that they are electric only. No water hookups.
We are going next month and we are staying at Spokane Creek Resort which is in Keystone. Another fairly centralized place. Your length is a factor at some places but most importantly in your travel. Wherever you book make sure you call them and ask about directions. I cannot stress this enough, DO NOT RELY ON GPS. There are roads through the Black Hills that will not accommodate your rig. On the Spokane Creek reservation email I received it specifically gave directions and said do not use GPS as you will end up on sections of road that have turns and too tight for most large rigs.
Here is where we are going. http://www.spokanecreekresort.com/