I would highly recommend getting the "build-in kit" for whatever brand microwave/convection you get or buy one that is designed to be built in.
In convection mode, convection micros get much hotter than the standard ones. When it was first installed, the temp behind mine got up to 200 degrees F after 45 minutes in convection mode.
I did not get the build-in kit for my first installation and went through a lot of hassle getting the temp behind the unit down to a reasonable level. I ended up adding a grill above the microwave and an exhaust fan blowing air through the side of the motor home. This involved a lot of work and cutting through the side of the motor home.
The second time I got the kit and the job was much easier. There was additional sheet metal in the kit that formed ducts to move air in from the front and back out the front. It also came with decorative grills that make the installation look like it came from the factory with the unit already in place.