75 Series Troop carrier auxiliary battery charge problem

Hi, I run a duel battery setup in my 75 series troopy which charges a 60l engel fridge and only a few small lights in the evening. The starter battery and aux battery are near new. The alternator was only replaced a year ago too.

The problem I have is that the alternator only seems to be charging the aux battery up to 12.9V max, even after I have been on the road for hours. My battery monitor is set to shut the system down when the aux charge drops down to 11.9V to save my aux battery from being damaged. Last time I camped, I drove for three hours and I parked up at 4pm. I woke up at 6am the next morning with a warmish fridge full of food. It was reading at about 8 degrees which means that the aux battery charge would have dropped down to 11.9 and shut the fridge off quite a few hours earlier on in the night.

Has anyone had this sort of problem?
Shouldn't the alternator be charging my aux battery to a higher voltage after I have been on the road for so many hours? Surely it should be reaching up to a high enough voltage that I can at least park up in the evening and keep a fridge running for one whole night easily.

Thank you!

Chris