Those typically go on the inside of the fridge
at the bottom to get a little circulation going to prevent hot and cold spots inside. I think James and Raz are talking about a fan that blows on or through the outside condensor coils. They are usually 12v. Sometimes solar
. On bigger roof vented Rv fridges they sometimes go in the roof vent cowling. I have seen one that has a roof panel and a small fan that snaps to the vent.
I think it's worth looking at how the vents are constructed first before going after fans. The venting in my U-haul
was awful and although it had a lower and upper vent, it was mostly blowing past the upper and venting between the trailer walls into the trailer! I made a guide for the top out of 3" metal duct tape beginning along the back upper edge of the fridge
and going up at 45 degrees to the top edge of the upper vent This helps to guide the hot air out of the upper vent, keeping it out of the trailer and preventing it from heating up the fridge box. I also added some Reflectix insulation on top of that. It's working so much better!