Don't know about KOA, you might be thinking of Lake Fairfax Park (Fairfax County - campground).
Click hereWe stayed there for a week many years ago with our pop-up. It was a great spot, as it was only about 2 miles from my wife's company headquarters (at the time, in Reston, VA) right after a company merger. WE got to visit the headquarters building, one of the few from Indiana who actually got to see it. This was a real treat.
It was also a good spot to branch out and re-discover Washington DC. I was Stationed at Fort Belvior, Virgina right after my wife and I got married, and both our kids were born at DeWitt Army Medical Center on Fort Belvoir. We went back and look up all the places we lived, (both on post and off), and went back to Manassas, VA and saw the house we lived in for about a year after I got out of the Army.
The campground, back then, was very nice (in my opinion), and can only have improved since then. I remember the water park there, and we spent a day there too. The kids had a great time. We (adults) had a great time, and it was because of the campground that made it all possible.