I noticed that the voltage on the batteries is all over the place when I drive in our campervan. A DC-DC charger is fitted from the factory, not a Victron. Is it possible that is why it does what it does? as soon as I turn off the engine, the voltage is stable on both batteries.
The DC-DC charger is a Schaudt Booster WA 121525 installed.
Where did you install the smart shunt, directly, and as the only, on the negative terminal of the leisure battery, and all the negative connections on the smart shunt “Load Minus” connection? If so, then you are reading fast fluctuating voltages from the running alternator, what should not be possible due to the Schaudt DC-DC.
When you start the engine, can you read the led status of the Schaudt? What it’s doing, etc.
One explanation is the DC to DC is drawing too much current from the starter battery/ alternator, the starter battery voltage then falls lower than the cut off voltage so the DC to DC stops charging, the starter battery voltage recovers and the dc to dc starts charging again. At one point the starter voltage drops to 12.7V. Make sure you have goog connections between the starter battery and the dc to dc.
Faulty alternator regulator or is this a Smart alternator varying the voltage.