Hi, I'm traveling in a Grand Caravan with the sto-n-go seats which make a nice area for sleeping. In cold weather I'd like to find a way to stay warm all night without having the car running for heat. I've looked at 12v plug in heaters but they aren't very big and may run the battery down overnight. There are also 12v plug in fleece blankets, but I'm not sure how effective they would be. Has anyone tried any of these?