Wanted to share that in ND, many if not most small towns have city parks that you can often camp in for free. Always good to check in with the police department, or at the local diner to find out protocols. Many of the tiny towns also have swimming pools, so paying to swim gets you a shower! Small towns in Montana also often have places you can camp, if you eat in a local establishment and talk to folks, you'll find them. Looking at it from East to West, our lodging was: Free city park in Glendive, RV park in Circle, Free city park in Jordan, Free park in Winnett (no bathroom after pool closed), Warmshowers in Lewistown, free park in Geraldine, campground in Fort Benton, and RV park in Great Falls.
There are two rest areas on the route through MT, both with world-class, amazing facilities, that people have reported camping at (overnight parking is allowed). One in Mosby, and one between Circle and Brockaway, called the Flowing Wells Rest Area. We stopped at both but didn't camp. Best water in Montana at the rest stops!
We rode the route from West to East in 2018, so not sure how relevant our experience is. Did the Lewis & Clark section from Lolo Pass to Missoula and around to Great Falls through Augusta, so that section isn't relevant to you, but then we went from Great Falls to Fort Benton, through Geraldine and down to Lewistown (still on L&C route). This was one of our favorite bits in Montana. From Lewistown, we were on the Northern Tier. Sweet little town park in Geraldine where we stayed for free. Be prepared that MT-200 has some rough spots where the road has so many frost cracks that you feel like you are riding on a railroad track. It is numbing. I echo other posts talking about the long sections between services on 200, especially from Lewistown to Glendive, and I worry about some of the tiny places, after the pandemic year--did they survive?. Additionally, we hit numerous severe thunderstorms during our crossing of Montana. I ended up making a list of ALL service stops across MT, and we would hop from one to the next, check the weather, and act accordingly. After getting caught in one especially bad lightening storm, we adopted the mantra "check the weather and believe it." People drive fast (we joked that the 70 mph meant that was the LOWER speed limit...), but are super friendly.