We have the microwave/convection oven combo unit shown here.

The louvers above and below the oven allow air to circulate around it as the oven manufacture requires for convection oven use. The vents do NOT recirculate stove-top air. It has no intake below it and has no fan. Feel free to click
HERE to see what the oven cavity looks like with the oven removed. It's just a simple wooden box cavity.
In recent years, RV manufactures are installing a new design microwave/convection oven that are also stove-top hoods which vent outside, much in the same manner those appliances do at home.
In our case with the older design, the control panel underneath the oven is the power vented cook-top hood with a duct to the outside.