I just got back from a nearly week-long trip to NYC, as it turns out. I spent two nights at Floyd Bennett Field in Brooklyn, my second time camping there, and then three nights at Croton Point as Floyd Bennett Field is only opening their campground on weekends as it's the end of their season. I'm not sure I understand the logic there entirely, and the web site is less than crystal clear that they do that, but nevertheless it's what is going on.
Liberty Harbor, which I haven't actually stayed at, is the most convenient location to most of the sights in the city. It's also the most expensive of the three, and has the smallest camp sites/parking spaces.
Floyd Bennett Field is very affordable ($30 a night, or I think less if you're retired), has few ammenities (dry camping at the edges of a very large concrete pad, a former sea plane base, with a dump station available), and requires more or less interesting city driving to get an RV to. The most straightforward route, via the Belt Parkway, is limited to passenger cars as are all the parkways. From the northish direction, at least, perhaps the best route is to take I-278 to Flatbush Avenue and drive Flatbush clear across Brooklyn; that's at least how I've done it, and survived to tell the story. From the campground at Floyd Bennet Field, it's about a fifteen minute or so walk to the bus stop at the Ryan Visitor Center, and then very roughly an hour to central Manhattan via bus and subway. The busses run every twenty minutes. An unlimited MetroCard covers both busses and the subway; the seven day variant is $33 and in my opinion a very good bargain.
Croton Point is much easier to drive to, being well north of the city and barely off of US 9. It's a 20 or so minute walk from the campground to the MetroLink train station, and about an hour train ride to Grand Central Station and about $12 each way (for non-retired persons). Taking the train to Marble Hill instead is about half as much money, and then a longish subway ride can get one to central Manhattan, at the expense of maybe twenty additional minutes. The campground is easily the nicest of the three; the sites (at least many of them) are full-hookup, and have vegetation between them and generally what one would expect from a public park campground. As was mentioned, it's around $60 a night.
Personally, I rather like Floyd Bennett Field, but it's not for everyone. I would definitely suggest passing on it if you don't feel adept at driving your RV in rather tight traffic or haven't yet gotten a great feel for where it is in relation to you when you're driving. Checking the clearances for whatever route you need to take to get there may also be important; I think the limit was 12' something at the interstate exit. (I don't recall any low clearances on Flatbush Ave itself.)
