The kids should like Gettysburg. They have the welcome center with movies and lots of info. May not like the entire driving tour so select the most interesting parts and not the entire route.
While in DC. Don't miss the Smithsonian Air and Space Museum at Dulles Airport. Also the one in DC. Use the hop-on hop-off shuttle all day pass.
If you want another option for NYC. We stayed at Newburgh NY KOA and took their tour bus into the city. Pricey but worth it as it includes some tickets like ferry to Liberty Island and to the Empire State Bldg. Look up the tour bus prices on the KOA website under facilities.
Getting your RV on the naval base might be a challenge. You will have to get a visitor's pass, provide insurance and proof of registration and they may search the vehicle. Also parking might be a challenge. They have a tour bus outside Gate 5 where you can park your RV and take the tour bus. Will post the link in another message.
