If its three way it must have a dedicated 12volt supply terminal, whether this goes straight to a battery or through a relay controlled by the ign/alternator (Which is the norm)when in operation is relevant, but it must also have a seperate permanent 12volt supply for the fridge ign and internal light Usually supplied by the leisure battery, an interrupted 12volt supply from the vehicle battery so as when ignition and engine is off it breaks the cct between VBand fridge 12volt heater this preventing the VB from total discharge. There will also be a 230volt connection of Pos/Earth/ Neutral for the 230volt heater operation. Most modern three ways will not operate without the permanent 12volt as this controls the electronics. If you study it carefully you will find all these terminals, your major decision is how you connect it to the VB so as it disconnects when stationary. Don't forget to take the 230v back to a suitably fused consumer unit and the 12volt positives should also be fuse close to each battery VB + LB.
To sum up there should be a 230volt terminal a 12volt interrupted terminal and a permanent 12volt supply the 12volt might share the same earth/neg supply.Phil
P.s. sharing neg I mean neg of both 12volt supplied not 230volts