These are from at least 5 years ago so things may have changed.
Fort Peck, Montana:
There is a RV park right next to the dam. Last time I was there they were charging $14/night for tent site, more for services.
Just to the west of Fort Peck: COE campground off of SR24; not well marked. A few sites on a windy hill. No charge the last time I was there.
Lots of boondocking possibilities both sides of Fort Peck Lake, campgrounds too. West side is nicer than the east side IMO.
Highway 2 south of Glacier National Park:
Old US2 parallels current highway on the south side. There are short access roads leading to it and boondocking sites (you are outside the park).
There are also parking lots for horse trailers along the highway.
Flathead Lake:
I've overnighted in boat launch parking lots a couple of times but that was 10 years ago. A number of paid campground around the lake. It was starting to get built up a lot around the lake so things have probably changed.
As mudbunny said, most towns in ND have a few RV sites in their city park. Most of them request a donation; some are starting to charge, mostly on the honor system but some have you register at city hall/police station (might not be open after hours). You have to check the town web sites as most aren't on free camping apps.
You will not be far from the North Unit of T Roosevelt NP. Boondocking opportunities in Little Missouri National Grassland next door (MVUM to see where).