I always turn on my fridge the night before traveling and run it on the way there. Most of these RV fridges are automatic 2-way, meaning (they need 12volt power for the controls) they need either gas fuel or 120 volt shore power to make it run and cool down and should sense which one is available...if gas only-should automatically light itself; if plugged to shore power-it should run on that. But all the ones I've seen must have the battery connected to be able to run on either source.
Bart and Tina
2001 Chevy 2500HD 6.0 Silverado 4x4-Banks Power
1997 Jayco Eagle 5er 285BHS-flipped axles