Mine came factory equipped, from Dometic, with a single thermostatically controlled fan, probably 4" fan. Mounted at the top blowing out the external vent. I thought it looked rather useless with no baffle or anything, but it actually moves enough air out that even with the fridge in a slide it cooled pretty well - and this has been a very hot summer for this area. It's pretty much always on, use a manual switch for your installation and likely not much different.
The internal fans are likely even more important, I made mine from 3 1" USB (so 5 volts) fans from a laptop. They are mounted right on top of the fins and powered from the light wires, no issues with interference with an overstocked fridge like the battery powered units that sit on the self but I've heard lots of people say they work fine too.