We went to Philly earlier this year. We stayed at Timberlane CG in New Jersey. Very nice CG with lots of room (and hookups) for big rigs and easy access off the highway.
We took the train into the city from the Woodcrest station - it was only 6 bucks round trip and it would be 5 bucks to drive over the bridge. They do have lots of parking in the city - there's a big lot right under the visitor area. We took the hop on hop off bus for a couple of days and walked all over the historic part all the way down to the art museum. Not at all scary. We were told not to be there at night though.
Edit... Here's a link to a similar
thread where we talked about Philly. Maybe it will help...