DC to DC issue

[amd6570]

My Rimor 2008 Ford Transit motorhome has a fuse less power supply that only has one cable going from the power supply to the leisure battery.
This cable provides a charge to the leisure battery when plugged into 240 mains hook up and when the engine is running, If not on mains hook up or engine stopped the same cable provides 12 volts from the battery back to the power supply to the lights, water pump etc.
I have a split charger but want to install a Lithium leisure battery, I know I need a DC to DC charger so not to burn out the alternator.
I have fed the DC to DC (Victron 12 12 30) with the cable going from the power supply to the battery, as when the engine is running and not on 240 hook up this is supplying 12 volts. The issue is if the engine is not running and not on 240 hook up there is no 12 volts going back to the power supply to power the lights, water pump etc as the DC to DC has no power.
I know I can just disconnect the split charger and run new cables from the starter battery to the DC to DC but this will render my 3 way fridge unable to run on 12 volts whilst the engine is running.

Does anyone know of a way around this issue please.

 

[RogerIvy]

A simple solution is to use a 5-pin relay. It means that some of the 30A of the DC-DC is used to run the fridge.

But first make sure it's not possible to simply disable the split charge relay without affecting the fridge. Then the fridge usually runs off the alternator and the DC-DC provides 30A to the leisure battery.

We've been told that Victron are working on a 50A DC-DC. That would make the relay option a better option - pretty much 10-15A to run the fridge and the rest for the battery.