I may be wrong, but I think the Master Switch can be in any position if you are plugged into shore power and running the fridge on AC power. I'm about 100% sure the Master Switch only controls 12V DC power.
Now, if you're driving down the highway, you are not plugged into Shore Power, so the fridge knob must be turned to DC for it to work on electrical power. Again, though, the Master Switch can be in any position, because the fridge circuit is not routed through it.
You could also be running the fridge on propane if you are not plugged into Shore Power. Again, I think the Master Switch is not a factor if you are using propane. It's not a good idea to runt the fridge on propane though while towing because the open flame is a hazard when you stop for gas at a filling station.
If you listen at the outside vent, you should hear the exterior fan running if the evaporator at the rear of the fridge is hot enough. That's one way to tell it's running. Mine has a light on the panel that lights up if it is not working properly. I also have a wireless thermometer transmitter inside that tells me if the fridge is working properly.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew