We consistently used the Backwall make up air model referenced in the study referenced below. I have used this when we designed systems for conveyor fryer lines in commercial applications.
So when it came to our food trucks as we had an open to outside wall I used basically the same concept but instead of bringing the inlet down the back wall we just put the fan along the floor. I used two or three bathroom vent vans with louvers set into the walls so the louvres were flush then put a screen over that. Frankly I don't remember how I calculated the cfm right now but they worked good for us. You need to be aware of airflow directly into any gas manifolds that may effect the performance of gas heating equipment.
I believe I referenced this somewhere in the forums in the past, but in case I didn't here's a useful study performed on the types of makeup air systems, that's what I based our designs on when it came to our first commercial design.
I does a good job of comparing the different types of makeup air supply, it also warns against the use of short circuit type vents, which I found there are a lot of out there.
I have used this when we designed systems for conveyor fryer lines in commercial applications.
As far as efficiency in a small open food truck, all I can say is its providing its own outside air from within the natural flow of air from bottom to top vertically thru the equipment and directly out the top, I don't think there's a more natural air flow than that. (That helped influence me when we took the plunge the first time into doing back wall in a commercial system.)
Keep any A/C you have focused on the people, your probably not going to efficiently cool the entire kitchen, but by keeping a blast of cool air on the employees it helps a lot. Speaking from a guy that has done a shift or two in Bangkok.
Referenced article: http://www.energy.ca.gov/...-04-10_500-03-007F.PDF