I would never drive a camper through NYC or Boston, ever! Boston has at least 2 circle routes around it, which will have traffic, but no tunnels and no bridges that are that scary. Let's face it, if you need to cross a river, you are going to need to cross a bridge.
A good route that goes way around NYC which we used, is to take I-84. We got there from I-380 in PA, north from I-80, as we were coming from the west - and yes, it will be a long way around. Beautiful drive, though, mostly through countryside, and not a ton of traffic. If you don't want to go that far west, you could use Rte 209 north from I-80; it follows the Delaware River on the PA side, and is a pretty ride too. If we can pull a 34' TT through there, you should be ok with it as well. Not the fastest, but scenic, and the bridges aren't scary.
If you were wanting to visit NYC on this trip, a good base camp, which is along that I-84 route, is the KOA in Newburgh, NY. Nice historic area along the Hudson, and the CG arranges bus tours to the city at least several times a week. Nice campsites too; some of them are nice & wooded, but still plenty of room for large rigs.
If you want to stay along the coast, then you'll just have to deal with the NJ/NY commuter traffic. Sorry, but that's the way it is.
The closest-in beltway around Boston is I-95, which follows the old Rte 128 route. Lots of traffic, but it's a big city. Middle of the day, the traffic isn't too bad, just during rush hour it can get busy. There's another interstate beltway, but it's quite a bit farther out. Don't take I-93 through Boston or you WILL end up in tunnels.
2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375