It will flatten a car battery in a few hours unless the engine is running its advisable to also use freezer packs
as well if you need to switch it off for any reason
Our camping gaz 12v cooler shuts itself off when the battery voltage drops below a certain point, it never therefore flattens the battery, but it will only run for an hour or so before this happens. On site if we need it we use a mains adaptor. It will only be of value if you are on EHU.
I have an Outwell electric coolbox and as long as you know the limitations of it then they can be quite effective at keeping things cool enough to be of use. I usually load mines up with pre cooled beer and maybe milk, cold meat etc the night before I go - run it on 240v overnight then plug it in to the car until I get to the campsite. It will be good for a couple of days before I need to take the car out for a run and chill the box down again. That usually only takes and hour or two. I also put a rectangular shaped snap box filled with ice in it the morning I go and that helps maintain the temperature even when the ice melts, the water still draws the heat from any foodstuffs.