Lots of great ideas! We have been to most of them and they are virtually all worth the effort to get there.
My 2 cents: 2 weeks would be a bit tight for Yellowstone - lots of long days to get there, or not enough time to fully enjoy the park. Some great things to see on the way, too, which could tempt you for future trips. If you foresee a 3-week vacation in your future, you might want to save Yellowstone for that.
If you want to do the history/city thing, you could easily combine NYC and Philadelphia, or DC & Williamsburg (and don't forget to include a day for Jamestown & Yorktown). We have been to the Newburgh (NY) KOA and it was a beautiful, wooded CG, unlike the KOAs we're used to. Lots of history right in that area, as well as the tour into the city. Our stay in the Williamsburg area was too long ago to recommend a CG; they've all changed since then. :)
If you want to do the countryside thing, northern Michigan is fantastic, and has been the focus of several of our more favorite camping trips. Michigan has nice state parks (most with electric hookups and showerhouses), and roads are pretty good even in the U.P.
Good luck with your decision!