Just wanted to provide an update as we're here now.
We arrived this morning a little after 1am. I was prepared (thanks to posters here) to be turned away and hang at the McDonald's or gas station if need be, however it wasn't needed. I arrived; the guard asked if I was returning and I said actually checking in today. He asked my name, took a few mins. repeated the number of guests on my reservation (not the same number as in my car) and said I was all set and to have a good morning.
I pulled into a parking spot and put our sun shade up for a little extra privacy and we were not disturbed at all. At one point I woke up and noticed someone was sitting in their car running a few spots from mine, but as far as I know they were not disturbed either.
Obviously anything could be different on any day, but wanted to report back the experience.
I'm glad we did it this way (even if we would've had to stay 4-5 hrs in the mcdonalds parking lot). I was way more awake, traffic was pretty much non-existent, and in my opinion definitely beats taking our chances at staying at some cheap roadside motel. I think someone had mentioned earlier about a 2nd driver...my DH didn't come; it was just my daughter and I so I was the only driver.
Certainly not the most comfortable sleep we've ever had, but doable. I did get up once to turn the car on for a few mins to get some air. - It wasn't terrible, but I didn't want to crack the windows as I know someone above mentioned bugs.
All in all it worked out well for us on thos particular trip. I would not do it if we had an additional person (at least not in this car). I'd only do it if each person had a "row" so to speak for themselves.