It sounds to me like youd be arriving around 530 am, and since nothing ever goes as smooth as you think, like an extra bathroom stop,, etc , you probably aren't arriving till more like 6 am. I see nothing wrong with nodding off until check in. However, someone said you may be able to check in as early as 6am. Especially if you aren't making a lot of room requests. Even if someone knocks on your window, you just say the truth, you just arrived and are waiting for check in. Once check in is open, if your room is not ready and you want to snooze a little in the car, I doubt anyone will care once people start bussling about.
Most likely your room wont be ready, but you''ll at least know what section it is in and can go park in that area.
I doubt people will think a whole family is dead in their car, and I'm sure people will be able to tell you all are sleeping, lol. Probably someone in the car will be antsy and not sleeping anyway.
This is not really a big deal.