I like to park in front of the beach to sleep in my car, thats how i travel for my country.
But in summer, even if you open all the windows of the car, the temps are annoying.
So i put a portable fan in the roof of the car, then i can sleep so good, no problems.
Just be carreful when you awake to dont touch the fan.
Step 1: Put a Inflatable Bed in the Back
Just turn down the back seats and then you can put the bed inside the trunk, its easier if you put the bed inside when is not full inflate.
Step 2: Put a 12v Fan to the Cigarette Lighter or to a Portable Battery
I prefer the portable battery to dont lose the charge of the car battery, but the consumption is very low.
Then put the fan in the roof with a office clip or a cord between the pillars of the doors.
And thats all, you can sleep all night long fresh and confortable