Battery voltage under load

Without getting into maths and battery specifications, the Engel will draw about 4 Amps and a voltage drop of 0.6v sounds a little high if measured at the battery terminals.
If however, you are measuring closer to the fridge, then the cable resistance comes into play and 0.6v drop may be quite normal.

Where are you measuring the voltage?
If you are using a multimeter at the pos and neg of the battery, then yes it is pretty low - I'd expect it to be around 12.5ish while the engel is cycling.

If you are measuring anywhere else, voltage drop in the wiring etc will be affecting the result.

If you are at all concerned about the condition of your battery, you need to test it properly under known conditions and with a known load.

When you have the time, take it out of the vehicle and get it fully charged with a known good charger.

Check the terminal voltage immediately you take it off charge and over the next 24 hours.

If you do not have a "proper battery tester".
Improvise yourself a load...one or two 100 watt driving lights and a reasonably accurate clock, used with your multimeter will surfice.

This will allow you do do a number of tests.

You should be able to get a full spec sheet for your battery off the manufacturers web site.

AND as others have mentioned, you are bleep in the wind if you are not measuring directly at the battery terminals.

As another matter...you can use your multimeter to check the voltage drops in your wiring.

ALL too often, wire used is too light.

cheers

Thanks for your responses. Yes, I am measuring the voltage at the battery. I will follow Bantam's advice and do some load testing to compare with the manufacturers data sheet.
Cheers
Yabbo