What brand fridge do you have?
I have an semi older Dometic RM2652 and this brand/model will re-activate the fridge when the power comes back up, assuming the button inside is pressed on. In my case, I have a disconnect switch on the A Frame header that kills the battery power inside the camper. At the gas station, I just turn off the battery disconnect, fuel up, pull away from the pumps and then a short ways away, get out turn on the disconnect and drive off.
I have heard, (do not know if they are all this way) that on a Norcold fridge you have to go back inside and press the fridge on button. There must be a relay inside that has to be reset upon power up. The battery disconnect switch would not work on this brand like we wanted it to.
If you have no battery disconnect switch, and your fridge brand/model will reset and turn back on when the power comes back on, then yes you can install a properly sized amperage 12 volt toggle switch in the incoming power line to the fridge. At the fridge vent on the side of the camper, take that vent cover off and find the 12 volt DC power line connection. Install the switch in that power line. You can then figure out where to mount the switch that is not affected by rain water and towing in the rain.
Hope this helps
John