The 'thermistor' is the temp sensor for the fridge thermostat.
IF you can not adjust temp set point via upper panel then moving the thermistor UP will make food compartment cooler.....moving it down with make it less cooler (about a 6*-8*F change).
IF you can change temp set point via upper panel......moving thermistor won't do much
The tip of thermistor should just be sticking out of top of clip..clip so be on far right hand fin.
Unplugging the thermistor.......no temp sensing so fridge will run continuously freezing the items in food section.
You say you are getting a fluctuation in temps while operating fridge on AC.
Have you tried running it on 'gas'? So you get the same fluctuations in temps?
If you haven't tried 'gas'.....try it and see if problem still exists.
And you do have the RV LEVEL...correct? Level is critical to fridge operation. It doesn't have to be perfectly level BUT needs to be reasonably level..level enough that you can comfortably walk around inside RV, full glass of water on counter doesn't spill out one side of glass.
Fridge needs good ventilation thru lower vent cover up/across backside of fridge cooling coils and out top vent for it to properly cool.
Is that side of rig heavily exposed to direct sunlight?
Is fridge in a slideout?
What Dometic Model?