I think most would agree that turning off at the bottle is the safest measure.
But if you think you need to have an appliance on whilst on the move, then you have to accept an extra degree of risk. Even if the appliance manufacturer doesn't have a restriction on such usage.
Roger, as you say gas is possibly more dangerous than petrol and with petrol tanks we have a secure filler cap seal for when not filling, but as for the tank take off that has to be open for obvious reasons, that's the risk we all have to take in order to drive.