I have a 3 way fridge, ie., a/c, LP, and d/c. The system's "auto" setting is designed to select the most efficient power source available. I don't use the d/c mode during travel, as the truck's alternator isn't large enough to keep up with the drain on the camper's batteries. I learned this early on while boondocking and had to use the generator to bring the batts up.
Should I run out of propane while traveling, the "auto" mode on the fridge would kick into the d/c mode and give me some more time to find a refill station or have one heck of a B*B*Q upon arrival at a dispersed camping spot. Plus, since my onboard generator is LP, it's pretty critical that I don't run out.:B