We live near the Lancaster area and frequently camp there.
I suggest Old Mill Steam campground, which is behind Dutch Wonderland, OR Beacon Hill campground, which is within walking distance of the town of Intercourse.
Neither one has many amenities, but both are clean and well kept.
If you decide to camp at one of these places, I suggest you reserve soon-they fill up fast.
You have to be careful when selecting a campground in Lancaster. This is a fairly spread out area and it can easily take a bit if time to get from a campground into the "tourist" area.
Both Old Mill Stream and Beacon Hill are close to everything.