Pace is important. A late September start and low mileage days could put you in NJ in mid-October. I've ridden that part of the route and also done some car camping there at that time. While not a certainty, you could encounter freezing temps at night in the Delaware Water Gap National Recreation Area/Worthington State Forest. One year I did the Black Bear Century that started in Delaware Water Gap, PA, which is on route. When I rolled out of a nearby hotel to ride to the start it was 34 degrees. But it would likely be for only one night. On the flip side, there could be some spectacular fall foliage up that way. Just watch out for bears. The stretch between Port Jervis, NY, and the I-80 bridge across the river into PA has a healthy population. In 2018 I saw a cub in camp at Worthington State Forest, which is a great place to camp. I'd try to avoid weekends as the place can fill up and you can only make a reservation for two nights, although they supposedly set aside some space for people arriving by bike, boat or foot. (It's near the AT.) Just call a day or so ahead of time and hope a fallen tree hasn't snapped the phone lines like one did the last time I was there. Also, they have a couple of bear lockers.
BTW...Do not ride the ECG in PA. Not much green about it. Lots of busy streets/road riding. At one point it uses PA 291, which is called Industrial Highway, and for good reason.