For a campsite you would be best making enquiries with sites as to what they would permit, as mentioned above sleeping in cars is often disallowed.
Though you may or may not manage to get away with it
One of my specifications on car shopping for my current one was the option to be able to fold down seats to sleep in with a mattress, roll mat etc and ended up with an SMax - but haven’t needed to.
My intentions would be when in a field at an event/festival and I would be in a bit of a ‘compound area’ rather than general camping areas
You would want some form of window shades, either the internal stick on ones (which will have gaps, or there are car door shades that slip on over the door)
If you aren’t tall then horizontal sleeping is a possibility across on the back seat or with some inflatable car seat mattress designs
Ive attempted to sleep cross ways once with a few of us due to space, it was an uncomfortable mess
If you are sleeping in a car, then breathing preferably needs a window partially open
Check your cars manual for the alarm settings. Mine has a setting that when turning off the engine I get a message to set the standard alarm or to disable the motion sensor - disabling the sensor allows me to lock the car and not set the alarm off
I have a basic back door tent, which allows the hatchback to be open and a ‘half tent’ over the back, this could be a compromise …. I personally wouldn’t sleep in it, but use it as a temporary shelter. Then take it down