We have done your itinerary several times on motorcycles, AS and 38' DP Motorhome. Now back to AS and doing it again. MT Rushmore KOA is a large, clean place to stay. Very busy and loud during the day but at 8:30 kids around us were so tired they went to bed. IF you stay there or near there, eat dinner at the Alpine Inn. Steak, potato, salad for $9.95 and it's not bad but the experience is worth it, their first seating is at 5:00PM.
Try Spearfish City Campground. Level, trees, stream goes through it. Very nice, well run and right on a long cement walking and bike path.
West Glacier KOA - excellent place. Going again this year on our way out to WA and OR.
We try not to boondock. Just like the convenience of our traveling motel. Did enough tent, canoe and kayak camping in early years to satisfy us.
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