Is there a reason you want to stay north of the city? There's a park & ride facility just outside downtown Philadelphia that has RV spaces. They have a free, frequent 24-hour shuttle that goes to a bus stop about a mile away. From there, it's a little over 2 miles to Independence Mall (Liberty Bell, etc.). You can take a bus, or you can walk (I did both, but I like walking in big cities).
The price is based on length of the RV: $30 up to 29', $40 up to 39', and $50 40' and up. Plus $13/night for the vehicle.
The pictures on the website are accurate. It's just spaces in the parking lot, but we were there for touring so it didn't matter, and it was great not facing a long trip home every night. I think it's a real find.
Campus Park & Ride