I heard there's one in Philly under the Ben Franklin bridge but I'm not sure - plus that's pretty far up.
The only thing I've found is to look for a marina list - and then visit them to see what the deal is - or even better - since not all marinas are listed - search on "pumpout station" + delaware river
pump.pdf" target="_blank">http://www.state.nj.us/drbc/
pump.pdf
New Jersey Pumpout Stations