Set the control panel on the fridge to "auto" & leave it there, this way when you have shore power or generator it "automatically" switches to electric, when unplugged it will change "automatically" to LP while traveling. The fridge will not run on 12 volt alone, unless by some off chance you have a 3 way fridge, but does require 12 volts to operate the control board. The fridge uses very little LP, the furnace though is quite the LP hog.
Others will say turn it off when doing this or that or going here or there, but mine is set & I have never turned it off in the last 10 years of fulltiming with NO issues, you'll have to make your own decisions.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless