Can't speak about motor homes, but our 5er microwave had 2 screws hidden under that compartment bottom. I had to use a 1" hole saw, turned by hand since there is no room for a power tool, to cut access holes in the floor panel. The screw bottom end may be seen by removing the trim above the microwave door. I estimated close enough to remove the top screws through the cut holes. Since we use the non-skid rubber matting in all compartments, the now-unneeded holes are unseen.
Our microwave now has a new blower motor installed and is working great. I located a motor by calling locally-owned appliance stores until I found one that salvaged used motors from discarded ovens, dishwashers, etc.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy