Many cars have tie down points, but it's difficult to use them to secure a vapour take off gas cylinder in its correct upright attitude. I have seen so many people lay cylinders down on their sides which which is definitely not a good idea, even when empty. Not only does it mean if the valve is faulty or not closed completely will leak liquified gas rather than vapour, and the "heavy ends" of the gas can be washed up into the valve gear which can then find their way into a gas regulator or even into the system pipework when the cylinder is connected.
Why would you want to carry a gas bottle inside the caravan when caravans have a dedicated external cabinet designed to secure and carry cylinders?