I've only used any of those centralized reservation outfits once. That's enough. They DON'T know the campground...some of the staff don't know campers and will put a 30 footer into a 20 foot back-in.....
The one exception MIGHT be the KOA system. However...I personally call the campground itself. Make the reservations there. When you ask (and you should) they will tell you the cancellation policy right then and there.
It also works when traveling, say coast to coast. Figure out where you will be in, 300-400 miles and callthe cg. around 10AM. (Miracle of cell phones)
Never a problem.