On an 18 year old fifth wheel you may find that you are limited by the size of the microwave opening. My suggestion is to remove the trim and take some measurements. Also look at the opening and surrounding area because you might be able to trim some cabinetry so a large micro or conv will fit. Also, another consideration is whether or not the micro/exhaust fan vented or un-vented.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054