You probably can only access through the bottom vent so this will limit where you could fit a fan. In an ideal world you should mount the fan horizontally above the burner so that it is blowing air up the back of the fridge, however the clearance behind the fridge may restrict you to a very small diameter fan.
Your only practical option may be to fit three 100mm or more fans to the inside of your bottom grill. The fans should cover as much as possible of the face of the grill. In this case the fans will suck outside air through the grill and blow it around the base of the fridge which will pressurise the space and therefore force additional air upwards over the fridge cooling fins. Ensure that the fridge is properly sealed to the habitation area as putting the fans in this position has the potential to blow gas/CO into the habitation area.
If you put so many fans in then the fridge may stop altogether, or maybe even overcool, it's a fine balance getting the airflow just right, one fan at the top is best and blowing out, covers all off to help natural air flow, to put fans at the bottom to pressurise the space may allow air to be blown into the hab area.
, do you have a chimney in effect rather than a rectangular top vent in the side of the van? if so you should still be able to fit a fan horizontally if you can get good enough access, maybe by pulling the fridge out of it's housing.