Moving thermistor on a Norcold N81 is NOT a cure ....nor will it make much of a change in temps.
Moving thermistor Up/Down will affect temps when fridge does NOT have adjustable temp settings..Auto Temp Controls
Once again I ask......what does flame look like?
Does the electric heater element work.....get hot?
How long have you had fridge ON?
Are you getting any fault codes.....light flashing etc?
See pg. 12 for Fault Codes
https://www.thetford.com/wp-content/uploads/2015/04/1297/Owners-Manual-Through-32306.pdf