Do be aware that getting into the constitution house, seeing the liberty bell, and a few other things in Philly require obtaining a (free) timed ticket in person beforehand. It's probably unrealistic to expect to get into those areas if you're only stopping for a few hours. (I do not recommend attempting to drive your RV in Philly, perhaps even more so than some other cities. Many streets are narrow, some extremely so, with quite tight intersections.)
In DC, besides the many excellent Smithsonian museums, you may want to check out the FBI museum/tour (which is interesting, but not up to Smithsonian standards IMHO) and the National Geographic Society museum (which is not free and a bit out of the way). I understand the holocaust museum is also excellent though I haven't seen it myself. There is a whole lot to see in DC; you won't have any trouble at all filling two days and then some.
Nighttime in New York (and elsewhere)—I might be tempted to stop by the TKTS reduced-price theater ticket booth and see what was available for the night. If you want more control, you can of course get non-discounted tickets further in advance online to just about all shows. Many of the shows unfortunately are not entirely appropriate for younger children, so naturally some discernment is needed. That said, seeing a good live show is a wonderful memorable experience.
If it's a clear day, the Empire State Building is an appropriately touristy thing to do and does have a magnificent view of the city from the observation deck.
