We stayed at the
KOA Philly South (KOA website) campground last year. Since we had a car with us (it was a part of a whole PA trip--Pitt, Philly, Washington) we just drove into Philly so don't know much about public transit from there.
It is a nice campground, however.
There is
The Campus Park and Ride which is basically a parking lot, but you can get anywhere in Philly from there (according to the reviews at that link).
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