Get a twin fan set like this, and fit it to the inside of the vent grill for the van with zip ties (make sure you fit it the correct way round) connect it with suitable wiring for a 12v system you can add an in-line fuse if you like, Now at this stage you can do one of 2 things, either wire it straight into the back of the 12v switch so that it only works on 12v or add another toggle on/off swith so you can also use it with gas/240v
If you add 1 at the botton vent to draw in air and 1 to blow it out you will really have the fridge working much better

