We made a trip in July that corresponded in part with the route you are considering. I will just mention a couple of campgrounds:
1> 7th Ranch RV Park is near the Little Bighorn National Monument. We passed by on I-90 and did not stay here because it did not fall into our overnight pattern. But we considered it, and I think it would be a nice place to stay:
7th Ranch RV2> We "swung over" to Big Timber, MT and stayed at the Spring Creek RV Park and Trout Ranch. It is right on the River and we very much enjoyed staying there:
Spring Creek RVFrom Big Timber, we went up US 191 to join US 87 just west of Lewistown, MT. then on up through Great Falls to I-15. We turned off West on Montana 44 to US 89. At a roadside park we found a marker which told of the experience of Meriweather Lewis and his only really hostile encounter with American Indians. Unexpected pleasure.