A refridgerator operates on the basic premise of taking heat from one
place and transferring it to another. In most cooling systems, this is
done by compressing gas, dissipating the heat of compression, then
allowing the compressed gas (often by this time a liquid) to evaporate
in the space you want to cool. This absorbs heat (and hence cools the
inside).

In the case of the Dometic fridge used in our Westy's, the place the
inside heat is gotten rid of, is the back side of your fridge (where
the fan is). If you have poor circulation of air then you won't cool as
effectively as if you have good circulation. The fan is supposed to aid
the circulation of the air. In point of fact a number of things can
hinder the air flow (and effectiveness of cooling). Blocking the vents
at the bottom of the fridge, or at the top will obstruct the air flow.
Blocking at the top is probably more significant.

I've often though of putting together a bank of small computer fans to
pump air in at the bottom and a larger fan to extract air from the side
vent to assist my fridge. Haven't followed through on this yet.