Marinas usually do pump-outs, since boats are not in a position to dump.
If you are not going to be hooked up, the RV will likely not be warm enough to use the plumbing facilities, so for that part of the year the dumping issue would be moot.
You need to find someone who is already RV camping on the streets in NYC, learn what they do to get around the limitations of modern RV equipment. What we had in the 1960s could do fairly well without electricity, and travel trailers of this vintage are often placed in fields and woodlots in rural areas to be used as hunting camps.
But if you get too modern, everything depends on maintaining an electrical supply, which might mean charging 4-8 hours a day to keep a furnace running overnight, or pumping water.
