When my wife defrosts our refrigerator, it may take 4-6 hours of 100% compressor
run to pull the temperature back down to where the regulator
kicks it off. As I remember, the Danfoss BD-35 compressor should remove about 300 BTU/hr, or cool down 30 pounds of contents about 10 deg F per hour with no heat leakage. It will also take about an extra 150 BTU to freeeze each pound of water
I haven't used a Waeco 12V/110V refrigerator, but the some of the control circuits on the Danfoss compressors vary the compressor speed (and amperage draw)depending on how much electrical
power is available. It could be that your Waeco unit runs the compressor hard enough on 110V (it assumes lots of power available) to get the unit to cycle off, but not hard enough on 12V.
Try turning the thermostat to the warmest setting and see if the unit cycles on 12V. If it does, then your problem may be not enough insulation