A word of caution:
I have replaced two microwaves with convection types, one in my last motor home and one in my present unit.
Unless the one you get is designed for a built in installation and exhausts out of the front, or your current installation has a way of exhausting through the wall I would highly recommend getting the "build-in kit" for whatever brand microwave/convection you get.
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.
Note that there are some minimum clearances in back and on the sides that must be met. You should make sure the convection micro you buy is small enough to allow for the proper clearances.
