We stayed at the Jonestown KOA last summer. Easy drive to Hershey and the Amish markets around Lancaster. The cg had just become a KOA, and had been making some upgrades. It was nice and they had some seasonal sites mixed with one nighters from the interstate and weekenders going to Hershey, and they were the best kept seasonal sites we've run across.
We got a river front site, which was really nice. Had a concrete pad and steps down to the water. They have tubes, kayaks, and canoes for rent and a shuttle if you want them take you up river to paddle back to the campsite. Kids had a great time in the river - it was big, but shallow and the water is slow moving. Might not want a river front site if you have small kids