We stayed here one night while traveling on Highway 12. This is a city park in Roundup, a small town along with a string of smaller towns along Highway 12. It is called Cow Bells
The cost is by donation that you place in a pay station near the entrance. There are flush toilets, a little dated but clean and functioning. There is a dumpster for your trash near the restrooms. I believe this is the campground for the Musselshell Rodeo.
The sites are spread out with picnic tables. You are sheltered by huge cottonwoods. While it was windy while we stayed, none of the branches fell. We were there mid September when the cottonwoods turn gold.
This was a godsend to us as we tried Deadman’s Basin but there were signs warning of rattlesnakes. Also, there was little shade there. There are few other public places to camp and in this part of Montana there is little public land.