I had one on the last MH and will probably install one this winter on this one. We have not had temperatures in the summer here in Montan that would need that additional cooling help. Next summer we will be in areas that get into the high 90s and low 100s so will need one then. I have used both the comercially available one from Camping Workd and 12 volt muffin fans. I prefer the muffin ons as they move more air and are quieter. The problem then is to find a thermoswitch at a reasonable price. If you do find them, you want one that will operate between 80 and 90 degrees, turn by 90, turn off by 80.
This summer I had a problem with my refer getting to warm inside. It appeared the two vent fans weren't working. I placed a 12" box fan at the accessory hatch outside. Made a BIG difference in the performance of the refer. Finally the fans started to work again so I could remove the box fan. It certainly won't hurt to put a fan in there and it will help quite a bit when the sun is beating on the outside of the refer enclosure.
Now my only problem is when will those fans quit again. ???