I haven't made that particular trip, but have toured around a reasonable amount in the area.
Besides NYC (as most of the other posters have suggested), I would avoid driving through Boston. Driving under Boston isn't too bad (in the tunnel) but propane is forbidden from the tunnels, so it's a no-go for an RV. Driving Boston surface roads are no fun.
A solution, if you wish to see the city (and it has a lot to offer), is to take public transport in. I've gone from the Alewife station most recently, which has a large parking garage (that would not be appropriate for an RV but would be for a towed or tow vehicle). Apparently sometimes getting a parking space there during weekdays can be a bit of a problem, but it isn't during weekends.
Some other suggestions of places to visit...the White Mountains of New Hampshire (Mt Washington, the Kancamangus highway, Franconia, etc.) are beautiful. Mystic Seaport, if you're into ships or boats or water travel at all. The Shelburne Museum is eclectic and unique (and large). There are many pretty areas of the Adirondiacks. New York also has some spectacular state parks, including (perhaps most famously) Watkins Glen and Letchworth State Park, both around the Finger Lakes region.
For RV parks, I've heard very good things about Lake George RV park, though I've never set foot there myself.
You might get more helpful suggestions if you provide a little more information. What sort of RV are you traveling with? What kinds of activities and sites do you like? What are you looking for in a campground/RV park? How tight is the budget? How much time do you have?