Your fridge a 2 way meaning it can operate on 120V AC (electric element as heat source) OR on Propane (LP gas as heat source)
It does NOT have a 12V DC heat element.
It uses 12V DC for controls......panel lights, circuit board, temp control and for the propane gas valve/spark electrode
It uses the 120V AC for the electric heat element.
The 'CHECK' light comes on when the main propane flame fails to light off and prove it lit.
That is ALL that 'check' light is for.....just propane flame.
Now as to why fridge is NOT cooling........
Could be a failed cooling unit. Cracked tubing that has allowed the coolant to escape and cooling unit to loose it's pressure.
Look for any signs of 'yellow powdery' substance....especially around burner area.
IF you find any.....cooling unit is done..fridge will not cool......new cooling unit or new fridge needed.
But it could also be a bad thermistor.......temp probe on far right fin in food section
Could be bad thermostat....part of controls
Could be blocked/obstructed air flow/draft across cooling unit
Could be due to being operated OFF-Level' and cooling unit has overheated....which eventually leads to cooling unit failure.
Obvious you don't have a good concept of how fridge works based on your posting that "flue is warm , nice blue flame----won't operate on any 3"
SO it would be worth the few dollars to have an RV Tech check out your fridge
If don;t want to spend the $$
Disconnect thermistor........run fridge on electric heater element for 12 hrs.
Food section will cool way down OR fridge cooling unit will overheat
