I understand the logic behind your request, just trying to assume what their reasons would be for denying such a request.
aaaaaaaand we all know what happens when you assume.
I took the liberty of asking the question direct from an online Disney agent. I wouldn't take it as a firm contract but you'll probably like the news. Here is a copy of the conversation:
Welcome to Disney! My name is Cassandra. How may I help you today?
you: Hi Cassandra, one quick question and I swear, I really am "asking for a friend." When it comes to the campsites (not cabins) at Fort Wilderness, is there a policy allowing (or not allowing) people who prefer to sleep in their car? If you get there too late to pitch a tent and you have no camper or RV, is sleeping the car allowed? I'm saying No.
Cassandra: Thank you for waiting. I will be right with you.
Cassandra: Hello and welcome! If you don't have a tent you may rent it at the resort, when you arrive at the reception area, they will be able to tell you the rates for their tents.
you: Thank you, I did let my friend know about the tent rental, and even that (I THINK) the tent would be set up and taken down for her. Her argument is it's a one night stay before starting a longer vacation with family who will arrive the next day, she doesn't want to set up a tent (she has one) AND she doesn't want to stay offsite or in a Value. This is her idea of a "scouting mission" for a possible future camping trip, but she doesn't want to set up a tent for the one night. This is why I'm asking. I wouldn't want her getting in trouble, but I suspect what she wants isn't allowed.
you: I can't find any info online saying it is or isn't allowed, but I'm guessing people have either done it or tried to do it and there must be some kind of rule one way or the other.
Cassandra: Well, let me find out for you, I believe you have to have a tent set up, however, let me make sure it is possible.
you: I appreciate it, thank you.
Cassandra: Is that for just one night or for multiple nights?
you: One night, then she is staying with a larger group in a resort.
Cassandra: Great! I asked and the answer is yes, she can just stay in her car for the night.
you: Great, pleasant surprise, I'll let her know, thank you.
Cassandra: My pleasure! Do you have more questions for me today?
you: No that's all I need to know today, though I have a trip of my own scheduled myself in a few months I'm sure I'll have tons of questions before then. Thanks for your help.