Fridge does not make cold it only moves heat to the outside coils, the more you can keep the back cool the better the fridge will work.
People use 12 volt pancake computer case cooling fans to help move air across fridge cooling coils as mentioned by Kevin. Just add a switch so they can get a boost in hot weather and not kill their battery
. Adding some baffles above and to the sides so that hot air can not collect and rising warm air pulls cooler air from bottom of vent across back of fridge.
Here is a picture of a fridge vent being set with butyl tape and rivets. 77 Scamp
in this case but these type vents are available from RV stores here is what scamp
a double wall construction? If so that vent install might be something that one of the members with more Uhaul
experience can advise on.