Venting and securing is the problem.
Most box store micro's vent on the side. You need one that vents from the top. Hard to find unless you buy a residential built in type. And they are costly.
I bought a smaller micro than the original RV micro opening that vented on the side hoping it would allow enough room to vent and it was not enough room to vent properly. The inside would steam up.
I'll raise my hand and admit stupid on on my first replacement micro:
I bought a smaller one to replace the RV one and when I went to leave I pushed it all the way back in the original micro opening believing it would stay there because it was so much smaller and there was this brace across the bottom from the original one......
OH how wrong could I have been!! 
I was leaving a sporting event on dirt ground and the first turn I made it came out of that opening, flew over the brace and hit the stove on the way down and broke the oven knob off and crashed onto the floor in a million pieces. It sounded like a bomb had gone off. 
I pulled over not knowing what had happened until I got around to the door and opened it. 2 squads pulled up right away too as they heard the sound. What a mess. :R