When I researched the microwaves that are sold as being for RV use I found that in truth they were cheaply produced units that had a front grill mounted and were junk - and expensive junk. With the mass of any RV the microwave that sits inside a cabinet is not going to get a lot of vibration and be damaged. The "RV" ones are hard attached to the mounting panel and their is absolutely nothing in the way of shock absorbtion being provided.
If I had a grill mounted microwave that died I would remove it and then measure the opening and buy a microwave from a discount operation like Best Buy or Home Depot and check the customers' reviews to see which brands seemed to last the longest.