Well, I don't know if this space is large enough to hold all the places you could see in S Dakota, Montana, Wyoming, and Colorado! Just the National Parks alone would make a great trip. Rushmore, Devils Tower, Glacier, Yellowstone, Grand Teton, Rocky Mtn, Black Canyon, Sand Dunes, and Mesa Verde come to mind right away. That is probably 6 weeks worth right there!
I could wax elequent on most of these. We have a trip to Yellowstone and GT planned this fall. We go to Rocky Mtn for day trips. We have a trip to Black Canyon in July. So here are some hints based on my experience.
Rushmore - there are lots of private campgrounds close by, plus Custer State Park. Depends if you want to dry camp or have hookups. My favorite is Rafter J Bar. The monument is a day's worth, Black Hills are worth exploring, and Custer SP is a nice day trip - lots of bison. Most will tell you to skip Crazy Horse. There are some caves south of Rushmore.
Yellowstone - haven't been yet but feel like I know it - I found a forum that is very helpful -
www.yellowstone.net - look for the link to "Online Forums" - a wealth of knowledge there. Fishing Bridge has water and sewer but electricity is off this summer for construction. All other camping is first come. Private campgrounds outside the park are available but driving in to the park each day eats up a lot of time.
Grand Teton - haven't been but will this fall. Great scenery and wildlife.
Rocky Mtn. - all camping in the park sans hookups. Estes Park has lots of private campgrounds but they have gotten a little pricey. A good alternative is Riverview RV in Loveland. Our TM group stays there on a regular basis. It is about 25 minutes to Estes. If you like to hike you can spend several days there. If not, a day or two may suffice. Lots of elk.
Black Canyon - take the boat ride in Curecanti Natl Recreation Area -
www.nps.gov/cure/planyourvisit/boattour.htm - then drive the south rim of the Black Canyon and take in all the scenic overlooks. Black Canyon has some camping with hookups, but in summer is it likely to be pretty warm. Blue Mesa has a few private campgrounds and there are some in Gunnison as well.
Mesa Verde - a must see if you are going to be in the SW part of the state. Stay in Durango - lots to do there as well.
Colorado State parks are very nice. Our favorites include Mueller, Golden gate Canyon, State Forest, and Ridgeway. Check them out at the state parks web site -
http://parks.state.co.us/Pages/HomePage.aspx. Weekends you will need reservations - if you can get them. BTW - Cheyenne Mtn is a great campground, but I wouldn't stay there in July. June and August can be iffy - heat and rattlesnakes would be the deterrents! But to spend a night on the way to somewhere or a couple of nights to see Colorado Springs it would probably be ok.
If you have any specific questions, let me know. We've camped in Colorado for going on 10 years so we've been or studied most places. Sounds like a great trip you have in front of you. Enjoy!
Alan