I have stayed at the off base park, "Gateway Famcamp", located just outside the main gate, every year for the past 6 years. I was assigned to MAFB and lived in Great Falls for over 20 years. I return every year to visit family. The concrete pads are mostly level...nothing is absolutely level. Water, sewer and electric are provided. There is a great walking trail just on the edge of the camp. A laundry and men and women's shower house is at the entrance.
The MAFB "Annex Campground" is located in a corner on base. This is the original campground and has not been updated in over 30 years. The gravel spaces are close together, mostly back in, and small. Showers are available at the base gym across the road and laundry is at the off base campground.
The base doesn't have an active runway so planes are not taking off at all hours but you will hear lots of helicopters.
Great Falls is the windiest city in America so be prepared for that. There is a lot of water nearby to enjoy on the Missouri river.