I have a thetford 3 way fridge and would like to switch it on the day befor we go away but don't understand how you use it in the gas mode as I think to have it on the battery would flatten it over night.
Does your storage site allow you to use gas? Most sites insist that gas bottles are removed or switched off when the van is left in storage, they certainly don't allow you to leave the gas burning overnight.
To operate the fridge by gas you firstly need to have the caravan level both ways. There needs to be gas in the cylinder and the screw valve on the top needs to be open (anticlockwise) to allow gas through. The isolation valve inside the caravan also needs to be open 'ON' to allow gas flow to the fridge. You need a fully charged leisure battery connected as this is the source of the ignition to light the gas. The 12 volt power switch needs to be 'ON' or 'VAN' which should register the battery volts at the voltmeter gauge. On the fridge fascia switch the fridge to 'GAS'. Turn the thermostat knob around about half way, hold it in to allow the gas to reach the igniter. If the fridge is electronic ignition just switch it ON till the gas ignites, if it's a manual igniter you need to keep pressing the igniter button till the gas lights which can be seen via a small round window inside the fridge. There may be air in the pipe that needs to be expelled so you need to continue trying to light the fridge but it helps to light one of the gas cooker rings prior to lighting the fridge just to confirm that the gas is operative and helps to draw the gas through the system. Once the flame has lit at the fridge, hold the thermostat knob in for approx. 15 to 20 seconds then release and the blue flame should stay lit then turn up the thermostat to the required setting. Remember to shut off the gas at the cooker. The fridge will take a while to get cold but the best place to check first is by putting your hand in the the freezer compartment to test if the fridge is actually getting cold. Once the fridge is operational by gas you can then turn off the 12 volt circuit at the switch as it is no longer required to function the gas ignition, however, if the flame does extinguish shortly after and needs to be re-lighted then the 12 volt circuit will be required once again for the source of ignition. As an additional check, if you put your hand over the external vent (directly behind the fridge) you should be able to feel a little pressure which is evident that the flue is releasing the fumes and that the fridge is working.
You will not be able to use the fridge on gas whilst the camper is moving, as that will blow out the flame needed to make the fridge work, plus it would be incredibly unsafe.
Why do you want to use the fridge on gas whilst it is storage? Many storage sites specifically ban the use of gas equipment whilst in storage. Some even ban gas cylinders on site.
Another option is to cool the fridge another way. I put lots of things in my house freezer overnight - Bacon, Any meat you are taking - cans of cider (Not beer or coke - they will explode) and Bread. I make sure I have plenty of cold beer in the fridge at home also.
When we are setting off I put them all in a freezer bag and transfer them to the caravan fridge. I put the fridge on 12 volts and set off. By the time we get to the campsite - I still have cold beer and most of the frozen goods are still pretty frozen.
Not familiar with a thetford fridge. But some (most?) fridges these days require a 12v supply to work the controls even when on gas. This doesn't take much power. This is different to cooling on 12v which will flatten your battery. I can only suggest that you read your manual or search one out on the web if you don't have the original.