I assume that you have flame if on Propane, and you stated that you felt warmth coming out of the upper vent, even without the ammonia smell could be the cooling unit, or circuit board.
My thought if it is on propane, and you have flame, then the cooling unit should be cooling even if you have a bad circuit board.
15 years is a long life for a RV reefer.
If the cabinet is still in great shape, if not the circuit board, I would replace the cooling unit.
RVcoolingUnitTheir tech support line is 901-337-9948, when I needed to replace mine in our old 5er, they were extremely helpful. Should be able to assist with troubleshooting.
