The specification will give the maximum energy consumption for each of the 3 sources - on 12 volt, the fridge runs all the time it's connected so the rating will be the consumption - 230V and gas will be thermostat controlled so less than the rated maximum.If I'm not mistaken, a 3-way fridge can be powered either by gas, battery or by mains hook-up? if this is correct then how do you know how much energy it's consuming? how would you go about measuring the energy consumption for each of the three ways?

Is your battery meter an "add on" or built into the caravan from new.