If your fridge is not cooling well on propane, but it is on AC, then you most likely have a propane problem. Check the flue, burner and propane pressure.
If the flue and burner are clean, look at the flame. It may look good, but if it in not "tall", say around an inch, your regulator may be going bad and not providing enough pressure. Low flame will not provide enough heat to properly cool the fridge.
Yes, it has happened to me. A manometer revealed low pressure, adjustment to the regulator at the tank helped a little, but not enough to fix the problem. A new regulator did.