So late that evening, I planned for my first Walmart overnight experience. I arrived at the Hayden Walmart about 8:00p, by which time the temp had dropped into the 80s. There was a designated section in the parking lot for RVs, and there were eight other rigs besides my own in the asphalt jungle. All was well until around 1:00a when teens congregated nearby to talk (loudly) and practice bird whistles. Yes, that's what I said. Bird whistles. Fortunately, they didn't stay long, and things quieted down quickly. I slept well until after 6:00a.
The next two nights I found nice forest service campgrounds, Lodgepole adjacent to Georgetown Lake southeast of Missoula and West Fork southeast of Butte just outside the west entrance of Yellowstone.