Most appliances, including this Propex heater are connected up to a gas bottle with a regulator. The way we used to change a gas bottle was to turn the gas bottle valve off and undo the regulator.
The older regulators that I am familiar with had a cone shaped nose with an O Ring type seal to prevent the gas leaking at the coupling. Now whether unscrewing the regulator everyday to remove the gas bottle is an acceptable industry standard I don't know. It would concern me that the continual unscrewing and tighting the regulator would/could cause premature seal failure with unimaginable results if the gas started leaking.
I did see a caravan for sale recently that had an outside tent. They had also fitted an on/off gas supply tap with a barb that a gas cooker/appliance could be coupled to. Looking at the setup, I would image that the user would just push the connector hose onto the barb, turn the tap on and light his appliance. I wasn't happy with that.
Whatever, I would prefer to get a vapour gas tank fitted inside the van, say behind the seats , with an outside filler to meet gas installation regulations. You shouldn't then have issues with carrying a 'loose' gas bottle in the van.
I don't have gas, but if I did, that is the way I would go.
Obviously they are not designed to work under water, but these would be "splash proof", meaning that if the odd drop of water dropped on them it wouldn't matter. It would probably be wise to protect them with a purpose made plywood box if you were worried.
Spruce