................ Do not believe their claims about their electronic speed adjusting thermostat. According to their claims this optional device will run the compressor at high speed when the engine is running and automatically switch to low speed when on batteries only. Not true. You must manually switch it from high to low. ...............
I have the Isotherm refrig unit with the optional electronic control. My experience is different than you state.
I find that the unit has good intelligence about whether it has engine/shore power voltage, or just battery voltage. Have you read the section on configuration of the dip-switches, and have you configured the unit to the behavior you want? It allows configuration for refer/freezer, target temperature ranges, min/max voltages, etc. When on higher voltage (engine/shore power) the unit runs at full speed and takes the box down to the target low temperature as rapidly as possible. When on battery power, the unit runs at a lower speed, and cycles within the broader temperate range window configured. You can hear the different compressor speeds (though the compressor is extremely quiet) sitting in a very silent cabin at the dock, with shore power on and shore power off.
I am very pleased with the unit, and glad I spent the small amount extra for the smart energy control controller.
Regards,
Andre