Maybe it would be a good idea to get some kind of meter and measure consumption? See here - https://community.openhab.org/t/new-binding-wireless-m-bus-techem-heat-cost-allocators. We are currently working on it and we will be able to plug wireless devices (battery powered). An obvious side effect which is necessary first - is modification of installation. But if you have a pressure meter on the tank - then you could try to read pressure and calculate remaining capacity of gas (with very low but still some accuracy).
Cheers,
Lukasz