One for the electricians?

1st question, how old is the battery??

As Dave said, is the battery OK? That would be the logical starting point here.

Another thing... is the fridge thermostat cutting the fridge off at temperature??

We usually make sure the lid stays closed for the last hour or two before bed time.. and last thing before turning in, we switch the fridge off and lock the truck up. I am always up and about at daybreak and first job for the day is to switch the fridge back on... We haven't lost any food/got sick etc so far from this and it conserves battery power for the daytime when we may need it most.

This works for us, but may NOT work for anyone else.... try it at your own risk.

Try bypassing the voltage cut-out switch.
You only have a relatively small capacity battery, after an hour of running the fridge has probably been drawing power most of the time for the initial cool-down and the battery voltage will be reduced.
When the fridge cycles again (starts up) the initial start current will be between four to seven times running current, which will induce a momentary voltage drop, which may in turn cause the cut-out to operate and you are caught in a loop.

Peter

Hate to tell you David, but 420 cca battery is pretty small to be running a fridge. Especially if you have only one battery. If you are serious about it you will need a second battery.

More money!

good luck

I have dual bat's the factory size batterys were very small but I managed to fit a 70NZZ in with no probs. They guy at the shop said there was no way I was going to get it in without modifying the car in a big way. I told him "Thanks mate, I'll take it and make it fit". Got home and it slipped in perfectly without changing a thing. It's a 720CCA 17 Plate jobby, heavy as sht but starts the car better than the two factory bats did by itself! ;-) So the isolator is in and working a treat. The beauty is I can now go and get another one of those beasts and replace the aux bat because it will fit! Long and short, don't take what these wallys at the shops say as gospel, everything is covering asses or too hard for them.

Unless separately charged by a suitable system, your car battery will be only about 70 to 80% charged. The alternator/voltage regulator system normally cannot charge above this. Your Low Voltage Disconnect thingy is designed to stop you deeply discharging the battery. Lead Acid batteries do not like to be regularly flattened.
Therefore you will probably find that you only have practically about 30 to 40% of the battery's rated capacity.
You could consider a Waeco Gel filled battery BUT make sure you upgrade your wiring and
/or install Blue Apple Charging Pack. Most dealers provide this latter item free if you buy the battery.

Check the open circuit voltage of the battery after it has "rested" unconnected to anything for about 12 hours with a digital multimeter. From memory 100% charged the voltage should be 12.65 Volts at 25 deg Celsius.

David the Danfoss compressor in the Waeco has its own low & high voltage cut out I think with the other one in line this may be causing your problem.

David. Is the Low Volt Disconnect adjustable? Some have a screw you can turn allowing a variable voltage cut out. It may be set too high. The battery is a little small for the job but should last for 1 night on low. Another test you can do is check the volts at the battery when its running then again at the fridge. You may have too much line loss due to the cable being undersized or poor connectors. If you are losing more than .2 volts in the leads it may need upgrading. My set up was losing .5 volts so ran 10mm cable including earth direct to the batt through 32 volt 2 pin plugs. Problem solved. If the Low Volt Dis is set at 11.5 Volts it's probably doing nothing anyway. I believe most Danfoss compressors have their own cut out at 11.6 volts . Cheers Craig..................

Just an adendum to this issue of running fridges that may help operations but is not intended to specifically answer the original Q

Firstly it is absolutely important that a fridge motor has as much air flow around it that you can possible provide, they must not be allowed to stew in their own heat. If the fridge is locked in the vehicle make sure windows are down to get a cross flow of fresh air, the heat is created from the transfer of temperature from within the fridge whist the motor is running.
Blow any dust out of the motor at least annually to keep the condensers clean.
If possibe, fitting a second fan motor (available from Jaycar) to blow across the motor really helps.
A conduit from the air con outlet to carry cool air to blow on the fridg motor may be possible.
And keeping a damp towel over the body of the fridge cabinet will help with evaporative cooling.

Any of the above will reduce the amount of time that a fridge motor needs to run to get the temp down to a desired level therefore conserving battery amps.