Something to note.
In my last motor home I changed from a standard unit to a convection type.
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 a 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.
A build in kit makes the job easy and the grill pieces supplied make it look like a factory installation.
If the existing unit vents outside then you might not need the kit. It is quiet though.
Clay WA5NMR - Ex Snowbird - 1 year, Ex Full timer for 11 years - 2004 Winnebago Sightseer 35N Workhorse chassis. Honda Accord toad.