We went to Philly earlier this year. We stayed in West Chester, which was a little bit of a ride. Beautiful country near the camp site. Visited Valley Forge as well. I'm not sure if there are too many places closer. As posted, I don't think I would drive a rv into town. There is simply no place to park.
We went to the historic area and had no problems. It didn't seem dangerous at all. If you go, get a PHLASH pass. This bus pass allows you to get on and off all day for a few bucks and takes you to more places you can possibly see in a day.