Jo-Anna,
We stayed at a small Forest Service campground in Cimarron which is about 10 miles East of the main entrance right on Hwy 50. Easy campground to get into and the spaces are sizable enough for the Montana. We thought it was a nice little park. Seniors with the Golden Age Pass pay half price. ($6-$7) They usually always have several spaces when we pass by there.
On Edit: Here is a photo showing part of the Narrow Gauge Train Display next to the entrance to the campground. The campground has no hookups. I don't remember if it has a dump station. It is not listed in our Campground Directory.
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