All of the convection microwaves I have seen are wide, more than many small microwaves installed in campers. I had a Sharp convection/MW in my Winnebago motor home, a R820 which was only 20½ inches wide, 0.9 Cu/ft and 900 watt, but alas, they are no longer made. New models are 24 to 30 inches wide.
Make sure that whatever you install is properly vented. Many times RV models have additional boxlike ducts that pick up the air coming out the back and route it up and out the front above the appliance.
Charles