We've always just used these cheap & simple temp gauges https://www.ebay.com.au/itm/LCD-Digital-Temperature-Thermometer-for-Fridge-Freezer-Aquarium-FISH-TANK-OZ-/273551026345?var=&hash=item3fb0e968a9
Work well enough to show that the 
cold! 
The one on the front left fridge is a temp
controller. They are cheapish (<$15 on ebay) and programmable for the temp that the compressor comes on and off at, as well as the +/- tolerance.
Mine can be set to something like +50 to -40 (not that the engel will ever make it to that. I set it to cool to -0.5 with a 3.5 degree tolerance, so the compressor kicks in again at 3 degrees.
I had some with 12 and 240V inputs, but they have disappeared from ebay. They also didn't like having 12 and 240 input simultaneously and the magic smoke escaped
, so some discipline and isolating switches were required.
The easiest way to set it up is to have a ciggy/merit plug running into the 12V input side and you plug that into the power supply.
The output you have a female plug that you plug the fridge into.
Drop the temp probe into the fridge (best to tape or zip tie it about 1/2 way up in the cool section) and turn the fridge dial to the coldest setting (also worth taping over the dial so no one messes with it).
Any adjustments to the temperature you now do on the controller. As long as the controller has power it will display the temp inside the fridge, the fridge will get power when the temp gets above the set point and the dodgy old dial is obsolete.
