Free Parking On Street Near Philadelphia International Airport?

We are taking a long road trip and need to rent a car. I don't think my sister-in-law wants to use her van because it might break along the way, and they don't want to use my car because it is a long trip and my car is too small and uncomfortable for them. Instead of renting where we live in the NY area, we will take my car to Philadelphia where it is a lot cheaper and rent the car there.

My reservation is for the Philadelphia International Airport, and I'm not sure we can get a car at any other locations in the area. I did this before, but rented from a place where I could park on the road across the street. It doesn't look like I can do this at the airport location. Since we have an extra car, I don't have to park my car within walking distance of the car rental facility and can park a few miles away if necessary.

I'm just looking for a safe and free place to park my car nearby. I need to leave it there for either 5 or 6 days. Preferably a place that is easy to find, but as long as I can figure out the address I can store it in my GPS.