Plainfield is in the middle of one of the biggest metropolis' in the world so no camping there. And you will have trouble navagating your rig around those city streets.
Spruce Run has a 24 foot limit on rigs and no hookups (they do have a dump station). This campground is a little run down but may be your only option:
Jugtown Mountain Campsite
1074 State Route 173
Asbury, NJ 08802
Some areas around Plainfield are worst than others so be careful about camping or leaving your rig in a Walmart.
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles