Next fall and winter, my husband and I are planning to travel the complete U.S. coastline in our truck & 5th wheeler (first year of retirement). We hope to camp in the fall somewhere within train/bus commute to New York City and travel in and out for several days. Anyone have any suggestions?

Fahnestock Park, about 60min north of NYC has tent/trailer camping sites. It's about a 10min drive from the park to the Cold Spring station on the Metro North commuter rail. It's about 75min train ride from Grand Central to Cold Spring, and about $10/person each way.

Cold Spring has some great restaurants, and the area is beautiful in the fall.

http://nysparks.state.ny.us/parks/info.asp?parkID=129

There may be something closer, but

There may be something closer, but I believe this is the closest (both to NYC and public transport) NY state park with camping.

To get to the park you cannot use the Taconic State Parkway (which cuts right thru the park) since no trailer are allowed. You'll have to approach on Rt 301 from either Cold Spring to the West, off Rt 9, or from Carmel to the East, which is right off I684/I84.

There is a campground in New Jersey right near the water - and I believe with a view of the Statue of Liberty. If you google it, or search below, you should find it. The last time I looked they charged about $70 per night to park there. And it has some sort of access to public transit into the city.

Don;t reco stayin 60 miles from the city - you will spend a fortune in money - and more importantly time - to get there via public transit. And obviously you can't bring such a vehicle into Manhattan - since there would be no place to park it.

Here's a page I found that lists a bunch of camp grounds and distance to NYC. The one nytraveler mentions is indeed closest. Liberty Harbor RV Park, New Jersey
http://www.libertyharborrv.com/
Here's the page with some other options.
http://www.hikercentral.com/metros/35620.html