We stayed two night in Hardin MT, staying at the KOA there. We visit the Custer battlefield while we were there. This is right between I90 and the start of I94 in MT.
When you are in Yellowstone make sure you drive the Bear Tooth MT212 highway out of the north entrance at Cook City MT. A very scenic highway but do not pull the camper very sharp curves. You can get off at WY296, Chief Joseph Scenic Byways which will take you into Cody WY. There you can visit the Buffalo Bill center in Cody WY, traveling on WY14/16 to the east entrance of Yellowstone which is also very scenic route to drive.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67