It's known as boondocking and basically it's RVing for free, without any water, sewer or power hookups. RVBasics.com says it's more accurate to call it over-nighting since many RVers resort to boondocking when they simply need a place to stay on their way to another location.
There are some popular boondocking hot spots, like Walmart parking lots, highway rest areas, truck stops, casinos, church parking lots, quiet side roads and wild life refuges. Camping World also allows over-nighting if the lot is not full or if you have an early morning appointment.
There are some things to consider when boondocking as well. According to RVBasics.com, RVs should always ask permission before setting up camp for the night. If you see other RVs, it may be okay to assume that an overnight stay is allowed, however.
RV enthusiasts are advised to avoid abusing the boondocking privilege as well. This means not overstaying your welcome and being considerate of the area and those around you. If you're staying at a rest stop, make sure to fill up your gas tank while you're there or eat at the restaurant to support the businesses and show your appreciation.
It's also a good idea to be discreet. Boondocking isn't a good time to take out your lawn chairs or barbeque and set-up camp. If that's what you're looking for, you should find a place that's meant for camping.
Some of the best ways to find a boondocking location include talking with other RVers and taking advantage of some great online resources. Boondocking.org lists options with GPS coordinates to make finding your location easier while AllStays.com has comprehensive lists you can use when planning your overnight stay.