You could work your way from Rushmore to Cody, WY - a fun little town. Then come in straight through the park towards Yellowstone Lake/Fishing Bridge RV area and on to West Yellowstone.
Here's the park's web site with maps and things to do:
Yellowstone Nat'l ParkBy the way, all national parks have an excellent web site such as this with tons of information.
While you're doing Yellowstone, don't forget about Grand Teton Nat'l Park which is just south. Both are completely different in looks and things to do. If you want hookups for the Tetons, Colter RV Park in the park itself is the place to stay. Otherwise there are very nice dry campgrounds in the park with water fill and dump stations. Gros Ventre is our favorite. It doesn't accept reservations but if you arrive before 2pm you'll get a site. There are 300 of them and it rarely fills. The same with Colter Bay campground which is right next door to the above-mentioned Colter Bay RV.