Fort Peck Downstream Campground is a real gem.  My son and I were there for four days, 8/29-9/1/2020.  My son is a quad and uses a power chair.  

Benefits:  This campground is run by the Army Corps of Engineers, and it shows.  Well paved roads, wide spaces, lots of room between spaces, laid out logically and well, and plenty of power.  The service buildings are clean and well kept, and one of the shower / lavatory buildings is brand new with easy wheelchair access.  Surrounding the camp are roughly five miles of mostly paved trails, perfect for early morning or late night walks.  Night time and clear skies bring out great star gazing.  The campground easily accommodates the largest rigs, with spaces large enough to handle 45' Coaches / Toads / and Boats all together.  There is also a 'group' space that could handle, I'm guessing, around 200 people.  Most electrical hookups are 30A; roughly 1/3 are 50A.  There are tons of things to see, and walking to the town of Fort Peck is ok for those in reasonable shape (I'm 57 and .. eh .. well rounded ... ); if you have a bike it is a snap to get there.  The very small town does have a grocery, restaurant,  and hardware store.  Glasgow is just 30 minutes up the road.  

Tradeoffs:  This is not a campground one can just show up at and get in.  Being a Federal facility, it is, of course, a Federal process to 'check in'.  One must, without exception, set up an account with the website recreation.gov ... note it is 'gov' not 'com'.  Next, one must pick dates and location ... a difficult process without any knowledge of the layout of the campground or what sites are close to what ... like playgrounds ... or bathrooms for those traveling without.  It is critical one does this BEFORE getting there ... as cell service is spotty at best and the folks at the gatehouse are not allowed to help or use their phone.  Don't ask how I know this, just take my word for it.  Trying to find cell service on a narrow two lane road in the middle of no where driving a 12 ton bread box is not a good time.  Just saying.  Once in, however, it is great.  This campground is on the high prairie ... it can be very windy, and storms come in fast.  If mountains and lots of pine trees are what you are looking for, this isn't it.  There are, however, lots of cottonwoods and tons of birds.  The campground is at the foot of the largest earthfill dam in the United States, and amongst other things, it produces power.  Which means that, especially at night, one can hear the steady hum of the generators outside.  Being male with selective hearing, didn't bother me a bit.  However, it might bother you.  In the summer, it can be very hot and dry, mid 90's are not unusual.  Night time lows are usually in the low 60's on down.  If you are there in the fall, it may well get into the lower 30's / upper 20's.  

Things to be aware of:  Make your reservation before you get there.  (Recreation.gov).  There is no water at the sites, however, spigots are scattered throughout the camp, if one has 50 feet of hose they can probably get to a spigot to 'top off the tank' ...

There is a dump station on the road into the camp, there are no full service sites.  The dump station is at most a mile away, and easy to get to if your teenager takes a twenty minute shower and fills the tank ... be aware that there are nice showers on site, at both ends of the camp.  The camp hosts are very friendly and helpful, even to old cranks like me who show up not knowing about making a reservation.  

For those who are in a wheelchair, this campground is a real treat.  There are miles of relatively flat, paved, walkways, including to fishing ponds.  The showers and facilities are handicap accessible, and one shower is specifically set up for folks in wheelchairs.