it uses a lot of power just to run nothing though. the guy with the unit at the end of the street dose camper vans up and one thing he recommends is to use a house hold fridge with an invertor, he says you wire it off of the termostat of the fridge to the DC out put of the invertor so when the fridhe wants to run it will switch on power to the invertor and when the fridge wants to switch off it will switch off power to the invertor (how he dose this i do not know! dose the thermostat on the fridge put out a circuit or no circuit when it tries to switch on, what kind of swithc on the DC side he has in mind i dont know)
That makes some sense, but has absolutely nothing to do with your problem, of running a 19v laptop from a 12v battery.
The usual 12v fridge for a caravan or motorhome, is what is called an 'absorption fridge', they work on gas, mains, or 12v. They are not very efficient, but it is the only sensible way to run from three different sources. They are so inefficient, that when on 12v, they do not even need a thermostat - they draw current all the time. On mains, or gas they do use a stat.
More efficient, is a compressor fridge, but they normally only work on mains power - via an inverter. The problem is, they still need a relatively large amount of current, so cannot be kept running for long, on a 12v battery, unless the engine is running, so the motorhome, must be quickly plugged into the mains, on arrival on site.
The guy at the end of the street, must use a fairly substantial relay, to switch the dc high current supply to the inverter on and off.