We stayed at a great private cg at Columbus, MT--right on the north side of the Interstate. Had the best shower rooms we have ever had--anywhere in the U.S. and we have traveled extensively. The park was very, very clean and neat, easy to get into and out of, and only $30 per night for hook-ups. I would highly recommend it. Plus--it had a great view of the Beartooth mountains to the south.
We have also stayed in a very nice RV park at Hardin, MT--close to the Custer Battlefield. Great host--nightly ice cream socials, reasonable prices. On the west side of MT 47 as you drive south off the interstate. A real bonus to a night is Hardin is to take time to visit the Big Horn County Historical Museum--especially if you have an interest in agriculture and its history in the area. This is a fantastic county museum.