I live within miles of the ferry, but I have never used it and I don't know about boondocking in the lot. A quick search shows the closest campground as Cheasequake Park in Matawan, about 9 miles away. There isn't a Walmart or Camping World close by the ferry either. The nearest Walmart is in Neptune off exit 100 on the GSP. If you are asked to leave the ferry parking lot, there is nowhere really close by to move to.
You said you would spend a few days in the city, are you camping somewhere in the area? Which direction are you coming from? If NYC is your primary destination, maybe Liberty Park campground instead of Belford? Liberty is as close to NYC as you can get, and there is transportation from there.
I would just like to mention, if you are in the Belford area, and like lighthouses, Twin lights is lovely and not far from Belford. Sandy hook is also close by basically over the bridge from Twin Lights and is a beautiful and historically rich place to explore. There is a lighthouse on Sandy hook as well. Both Twin lights and the Sandy Hook lighthouse offer beautiful views of the NYC skyline and Verrazano Bridge.
Christine & Keith from NJ
2011 Tiffin Allegro Bus
Travelling with our 3 kids and our dog