Cherry Hill - yes. The convenience of the Metro is hard to beat. Favorite spots in DC for us were Washington Monument, Nat'l Air & Space Museum, Arlington.
I-68 - as others have said there is a standard for grades on the Interstate system so grades should not be a problem. I-68 runs due east-west cutting across the Appalachians at about a 45 degree angle. You're either going up or down.
If you have the time Gettysburg is definitely worth the trip. Below DC is Mt.. Vernon, also worth the trip (you can do Mt. Vernon in about half a day). Prince William Forest Park CG is nearby - it is a nice little park. Sites are snug but we enjoyed our stay very much. Enjoy & safe travels.