Back in the day, this kind of engineering was part of how I began my career... though submarines are a bit more complicated. But the tight packaging is nostalgic, and the basics are the same. So here it goes.
The fridge is designed to operate in an "infinite" volume of ambient air, continuously renewed by simple convection. The onboard fan simply partakes of that easily accessible ambient air. The manufacturer will have a specification for this, in the form of a minimum air-gap on all sides. Find this. Follow the spec. That little fan is certainly NOT designed to move the air very far. Just far enough to dump the heat outside of its own box.
You do not need to use a second fan. In fact, you could quite easily cobble together a fan-vent combination that made things worse (by creating dead air spaces in the last places you want them). Not a dig; this is a common engineering error. And you are always going to be fan-function dependent, even if properly designed.
Instead, ensure an unimpeded spec air-gap surrounding the fridge that passively flows a very high volume of ambient air, with the fridge in place. That means BIG vents, both as low as possible in the airspace, and at the very top. Think minimally interrupted continuous-length vent, all sides of the fridge, with an opening width of at least twice the air-gap spec. I will not get into the details of passive flow induction, but equal vent areas top-and-bottom is generally ok (yes, you can do better... but it's complicated). The vents can be angled 90 degrees to the direction of flow above or below, but straight-through flow is always best.
You should not rely on vehicle movement to create pressure, just the opposite. In fact, all vents should be shielded from capturing the wind. Now that's counterintuitive - why? Because the wind may just be blowing in the exact opposite way that you need, stalling air flow and creating dead air space. That is easy to do in a narrow, confined space. Nearly impossible in "infinite" ambient. You want the heat generated by the fridge and discarded by the fan to help drive the flow, just as it would do in ambient.
So: all the vents must be designed to stall the wind, creating no pressure on the actual vent opening. You do this by layering the vent space; the vent opening is behind an exterior shield, and the space in-between is open to ambient in all directions. A functional example of this type may be easy to recognize - the mushroom shield on most well designed vehicle snorkels. Since your fridge vents will be highly elongated, not in the round, it will look very different from a snorkel. But the function is very important.
Your idea is a good one, with very efficient use of space. I hope this helps.
