The pink wire go from alternator to house battery on Bavaria ( at least until 2010 models?)
In case od installation of a DC DC charger Orion Victron that connect alternator only to engine battery, the pink cable must be connect to engine battery?
The alternator must be connected to a lead-acid battery, plus wire to the plus side of the battery minus wire to the minus side of the battery. I don't know what color wires you have.
I assume that you intend to connect the dc-dc converter to the lithium battery. In this case, the dc-dc converter is located between the lead-acid battery and the lithium battery. It charges a lithium battery from a lead-acid battery.
Check if you have a splitter, which split the electricity between the engine battery (lead-acid) and the house battery (lead-acid) in the new scheme, you don't need it and it "uses" electricity unnecessarily.
Using a DC-DC charger means that your starter battery will be charging your auxiliary batteries through the Victron.
The pink cable is the voltage sensing cable of the alternator.
You will want to keep your starter battery optimally charged, so that it can optimally charge the aux battery.
Therefore, yes, the sensing cable will remain connected to the lead-acid starter battery.