xsilvergs graphs are clearly those of a smart alternator vehicle . That confirms something a dealer told me recently about ducato's fitting a smart alternator on recent new builds . I fitted an Ablemail AMC12-12-30 on Vw t6 with smart alternator and it worked really well .
The other factor to consider is the 12v power of a three-way fridge. Of course you have to stop the SCR connecting the batteries together when the alternator kicks in if you have a B2B, but normally it will still switch the fridge power on (though my fridge has a separate D+ wire, which makes the change different)
I power the fridge from the starter battery, not the habitation ones, so the B2B charger is basically just charging, and the fridge runs off the alternator. The amount of 12v power the fridge takes when the engine is off is minimal.
That's as good as it gets without a B2B charger.
You fit 100Ah of leisure battery. You can only charge it to 85% full and only safely discharge it to about 50% full.
So your battery has a total usable capacity of around 35Ah. That's 438 watt hours: the equivalent of 31 grams of LPG, costing 3.4p