Fridge is running down my battery

Hi all,

My problem is I am by no means a sparky so I think I may have done something wrong. What I have set up is in the back of the ute in a canopy, I run a primus mammoth 65 litre fridge. The issue is that parked at work, in full sun, the fridge draws the battery down so low that it shows ERR1 which the tech tell me means is generally consistent with lack of power to support the fridge.

As it is running on the back of a ute it is running in hotter conditions than if it were running elsewhere, therefore I assume it will use considerably more power in this application. It also runs out of power when I park it in the open at 2pm and return to it at midnight, the tech says that running out of power during the night tells him I may also have a power supply problem to the fridge.

Can I check the math and set up with this knowledgeable group before I spend a lot of money I don't have.

Specs are: average current draw for the fridge 4.6 A power input 55W, compressor is a SECOP BD50F. Fridge is set to cut out at 10.2 volts. Average Consumption: 0.89A/h
The battery is a 130 aH AGM which is fully charged each night using a Microprocessor controlled battery charger. When driving the battery is maintained by the redarc BCDC125-LV in car charger which is located in the back of the ute also. Twin core 8 B&S cable is run back to the battery/alternator under the hood in accordance with the wiring diagram and 6 B&S from the battery to the merit plug (<2metres) the fridge connect to that by the 2 metre long cord.

Assuming hot conditions in the back of the ute what would the effect be on the aH usage with the fridge shutting down at 10.2 Volts.

Any help would be appreciated.