Rodney, winter covers are only meant to be used when the temperature is below about 8 degrees and are meant to improve the efficiency of the fridge.
The fridge should be sealed around it rear edges to stop any exhaust gas from entering the living area, but depending how old the van is, it may not have been.
If the van is fairly new, it "should" be sealed but there will be a gas drop vent in the floor under it. On no account must this vent be obstructed.
If the draught is really bad, you could fabricate a 3 sided shield to fit under the van to reduce the problem.