NYC, DC, Boston: My kids have not seen NYC or Long Island (where I grew up) yet. Would love to take them. Each of these major metro areas is within 8 hours. When we did DC a few years back the hotel was a place to sleep.
The one thing to be aware of with a B or any motor home is that cities pose a problem not encountered with a car or a trailer towed by a car that you can unhitch and travel in. The problem is parking. Cities are now doing away with open parking lots and parking garages are taking their place. A B has a minimum clearance of 8' 9" and that eliminates indoor garages. This means when visiting NYC, DC, and Boston you have to find parking outside the city and commute - generally a distance - by train or subway - to visit the city. Boston and DC are problems. NYC has a campground right across the river in NJ with a train stop a few blocks away, so NYC is fairly doable - but I would never drive my B into Manhattan, and certainly no larger RV. Of course, Long Island is wide open and you can come and visit here anytime with plenty of places to find outdoor parking. So, in terms of travel to cities, the trailers have an advantage of unhitching and driving your vehicle to where you can park just about anywhere.