Duke 89 wrote: it dont exists yet but it might be posible to make and what would you pay for it
I have no idea why you'd get the idea that it doesn't exist. There's hundreds. Did you try searching for "tent cooler"? That's how I found the first couple of hundred different designs.
Naturally it already exists and it cost $40. Doesn't use that much power either. The downside is that you need to have ice in plenty supply to actually cool the air.http://www.kooleraire.com/
If you want to spend a little more this is a less DIY model that hold everything.http://www.batterysavers.com/person-aircondi.htm
Most low cost portable AC units work on the water/ice cooling principle which means you must have a steady supply of very cold water or ice but compressor/peltier based units does exist. They naturally consume much more power in the process and you should expect a car to last more than a single day if it's really hot. Maybe not even a full day.
It would be fairly simple to make a more effective unit than those that exist already for a far lower price. All you basically need would be a small inflatable kids swimming pool as many have in their camp. Fill it with good amount of water. That water will be very cold in the morning and even into the afternoon if you have it set in the shade of the gazebo. Then all you need a water heat exchanger, also known as a water cooler for the overclocking community, a 12V pump, a 12V fan, some garden hose tubing and a battery. Wouldn't use much power and one swimming pool could easily hold enough water to cool 3-4 tents for several hours. Especially if you put a reflecting (white) tarp over your tent so that it doesn't absorb as much heat.