On Propane, if the flame does not ignite or blows out, in general there is a limited window of time for the automatic re light to operate, like maybe twice or three times, then it simply shuts down. It must then be turned off to reset it.
The spark lighting system takes very little power to operate, BTW.
I would look for the reason the propane flame goes out, and eliminate any use of 12 volt
power for the heater. I am surprised your unit has a 3 way refrigerator to begin with. My 2014 20' FC does not.