A friend new to the RV travel lifestyle recently discovered a website claiming to be "The Most Accurate List of Free Overnight RV Parking in the US". He reports his checking out of a small sampling of the listed sites in the PA/MD/DC/VA area was spot on. One must pay a fee to subscribe to the service, which is updated by on-the-road RV-ers, but the fee is < a night's camping fee most anywhere. There's a free demo screen which looks detailed and useful if it's an accurate depiction of the majority of the data on the site.
Although listed for RVers, seems as though non-RVers might also avail themselves of these facilities. These are apparently places to SLEEP for the night, not campsites where one drops the awning and sets up the grill and camp chairs in a circle.
Check 'em out at this web site.