In my last trailer (Springdale), I replaced the microwave. Got it from Wall Mart. It was red, it was pretty! It was a little smaller than the original, but that was OK, it was still bigger than the one in our house!
The microwave was above the stove. I drilled 2 holes through the bottom of the cabinet and ran 2 screws through the holes that screwed into the screw holes for the feet of the new microwave. The screws were longer and need to go through the wood, through the foot and into the base of the microwave. It worked beautiful.
I then took the old trim, which did not match, and reattached to the wood of the cabinet. It was "close", close enough it looked good. The top and bottom filled in pretty good, but the sides had about a 1 inch gap. But it still looked good. It worked perfect until we sold the camper.