Based on the time of year you are looking at, Upstate New York and New England would likely be more comfortable. Virginia and the Carolinas may well have daily high temps in the 90 to 95 degree F range (32-35 Cel) with high humidity. New England can be hot as well but is normally 10-15 degrees f cooler.
RV rentals up there, personally I dont like driving in that type of traffic, thats why I mentioned going to another city for the RV rental. RV campsites are also at a premium and therefore pricey there, by the time you pay for RV rental and site for the nights in NYC I believe you could have easily stayed in a decent hotel, and would already be in the city.
As you mentioned Cruise Am does have some locations in the area. Poughkeepsie is one and is about 80 miles north of the city, and and puts you on the edge of some nice Upstate areas.