Allowing just a half day to tour Philly simply will not work.....you need at the very least one full day.
I would highly suggest you make a two night reservation at
Oak Grove Park. The park is in Hatfield, Pennsylvania and just a few miles off the PA Turnpike. Very nice, clean, level sites.
To visit Philly the train station in Lansdale is an easy 15 minute drive from the RV Park. If you are a senior citizen you can ride the train for one dollar into Market Street Station. The train ride is about 50 minutes. From there you are a short walk, about four blocks to the Historic District. During the summer months a bus called the Philly Phlash runs a continuous loop to all the visitor must see places. If you are a senior citizen the Phlash is free of charge.
If you decide to stay at Oak Grove Park pleas feel free to send me Private Message and I would be happy to give you more details. II live just a few miles from the Oak Grove and we frequently take the train into Philly.
I would not recommend taking the RV and parking it anywhere near Philly in parking lots.