From a practicality standpoint, you should be glad you don't have the factory microwave in there still- they were inefficient monsters. That said, the opening you have is probably larger than the typical opening for the RV microwaves now.
I don't think you necessarily need a microwave from an RV dealer, but sometimes they have deals that make it worth it if they have a take out from a new unit. You may be able to get almost any microwave from a store and then you'd just need to order the cabinet mount kit for it from a dealer or the microwave manufacturer. It's basically a few painted metal brackets that screw onto the microwave that allow you to screw this bracket to the face of a cabinet. You'll probably have to add a little wood to the top/bottom of your existing cabinet to make it work.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR