If same symptoms using electric or propane
(Freeze gets cold/stays cold------food compartment doesn't cool down/stay cool/warms up)
then there is an obstructed/blockage in cooling unit tubing between freezer low temp evap coil and food compartment high temp evap coil
Course that is IF fridge being operated is level and ventilation across backside of cooling unit is not blocked
IF you have the GRAY encased control module (lower board) then you can force cool the fridge by unplugging the thermistor leads from P2 on the board and then run fridge for at least 12 hours using electric element.
Food compartment should cool down...if not cooling unit is bad
If you have the 'flip cover' control module then you can not unplug thermistor and cause fridge to force cool.
But using a new thermistor you can unplug the old thermistor leads from J3 on the board ---plug new thermistor leads back in and hang thermistor out lower vent cover/access panel. Run fridge at least 12 hours on electric.
Food compartment should cool down..if not cooling unit is bad