Weird temperature reading using thermistor on MKS GEN L v1.0 AUX-2 analog pins
Thermistors, commonly used as temperature sensors on 3D printers and other DIY things, work by changing their internal resistance depending on the temperature.

On the analog ports, the microcontroller can determine the voltage of the connected signal.

To actually measure the resistance - and thus temperature - of the thermistor, you need to build a voltage divider:



Most, if not all, 3D printer control boards use a resistor of 4.7 kOhms and a capacitor to build this voltage divider:



You'll need at least this 4.7 kOhm resistor to get your thermistors working on any analog pin, if the three thermistor ports on the MKS Gen L don't suffice for you needs.
You could also use other temperature sensors, especially for the case, like the DHT11 or DHT22, however I couldn't find whether they're supported by Marlin yet.