So, only the one wire.
Have you got a battery charger with an ammeter. The thing should take about 6 amps, and polarity is not important. If not then you might want to try the mains, but it may go bang.
There is a risk that it is just 240v and gas. I recall this was usual for 2 way fridges, ........ but the label says 240 and 12.
So there is a second wire. take the back off and find it. It may be that the 12v heater has blown, leaving only the 240 volt heater. You could buy a 150 watt 240 to 12 volt inverter, as its only rated at 75 watts.
BEWARE: Our chest freezer only runs at 0.7 amps however the startup current is 8.3 amps meaning 1992 watts at startup. We are having to buy a 1500W (2500W peak) invertor just to cope with the initial kick!!