Sleeping bag for car camping ?

What kind of sleeping bag do you use for car camping in the summer ?

I will be driving across Croatia from Zagreb all the way to Dubrovnik and car-camping along the way this summer. This will be my first time car camping, as I'm more used to backpacking. I don't wanna bring a sleeping bag that's too heavy/warm, but I don't wanna be cold either. Can anyone share their experience car camping in Croatia in the summer? Does it get very cold at night?