After further thoughts I think you're right David. The fridge will consume around 10-12A so the 30A B2B would be able to 'keep up' easily even if the fridge was connected to the leisure battery. It appears the alternator is the component that's not doing its job properly.
Would be interesting to have a DVM connected to the input to the B2B while driving with a decent load just to see what the cab battery voltage is getting up to . Pity there's no option to disable the 'smart' bit from these alternators too.
I may have came across the wrong way here with my initial post.
I was more looking for a solution as to how to get the best from these annoying smart alternators.
This is my first van with one, and I have been trying to work out how to manage the situation.
As David said we have moved on from it should be the cab battery, to something different.
And horses for courses comes to mind.
If charging your batteries is your primary concern, then minimise current usage whilst driving by running the fridge on gas.
If not then run it on batteries.
But the time of year comes into this also.
What may be right for summer, may not be right for winter.
People have ruined lead batteries with smart alternators, completely draining them in a single journey.
I have found running the fridge from the vehicle battery has helped the leisure batteries, and the vehicle battery still gets a charge.
But this may not work for others.
I am still trying to find the best solutions, and monitoring whats happening.
I intend to contact Fiat to see if they can adjust the smart alternator, but I don't think they would do this due to emissions.