I don't have a corporate code, but one of the reasons I like National is because of their one-way rates. In the DC area, my normal EC weekday rate is about $77-$80/day regardless of pick-up/drop-off with unlimited mileage, and I've done every possible combination of DCA, IAD, BWI, and MDT. PHL is in a different region, so the one-way rate does jump up pretty strongly there, but your one-two-free days work on that rate.
A couple of weekends ago, I had a BWI-DCA (work), and then a DCA-BWI (lesiure). The DCA-BWI weekend rate for 2 days ($19/day) was a better total than the rate for 1 day ($50/day), and the computer automatically figured it out for my reservation and charged me for two days instead.