In the resort parking lots, or more specifically, in our Honda in the resort parking lots.
Let's face it, the only thing to do when not in one of the parks or other guest areas is sleep. I don't feel inclined to pay for a hotel room just for 6 hours of sleep time. I have found our vehicle to be completely sufficient. And I have yet to find a Disney resort that does not have an accessible restroom near the lobby - which is perfect for brushing teeth, sponge bathing, and other bathroom needs. Sunshades and makeshift curtains provide for adequate privacy.
The benefits of in-vehicle-slumber-parties? Obviously, eliminating the cost of hotel room fees. Does a hotel room have a moon roof that allows for you to sleep under the stars? No. Does a hotel room give you access to a social network of friendly teardrop-trailer-toters? No. Does a hotel room provide the instant mobility that allows you to change the scenery each night at a different resort? No. ...And the goings-on of a parking lot in the middle of the night is often more entertaining than hotel room cable. Room service versus a 5 minute run to McDonalds... enough said.
At first we thought that security might be an issue, but so far everyone has given us our privacy and left us alone. We have also met others with the same idea. Granted, car seats are not quite as comfortable as an actual bed, but given the cash we're saving, it works just fine.