SHARP MICROWAVE OVEN REPAIRWe RV'ers really depend on our microwave ovens.   As D owners, we REALLY depend on our microwave because it is also a convection oven.   So, when it breaks it is a MAJOR inconvenience.
Most failures are due to blowing a fuse or a failure of the microwave-portion power supply.   Replacing the fuse is easy, as is determining if you have a power supply problem.
The first thing you need to do is to remove the microwave.   First, open the cabinet door above the microwave and remove the two bolts on the floor of the cabinet.   These bolts attach to the top of the microwave.   Next, unscrew the cord protective cord collar so that the ac plug can pass through the hole when the microwave is removed.   Now, it is best to have a "helper" actually remove the microwave.   The microwave is held in place by a metal panel that is secured to the coach walls.   The rear of the microwave rests on a lip on the bottom of this panel, and the microwave itself is secured to the panel by two long bolts that are accessed by removing the two filters on the bottom of the microwave.   The bolts are near the front of the opening after you remove the filters.   These long bolts go through the body of the microwave at an approx 45 degree angle to attach to the wall panel - this is what holds the microwave in place.
After you have removed the microwave from the wall bracket, you need to remove the big metal cover from the microwave.   This is a simple operation as it is just attached with metal screws.   As this cover is somewhat flimsy, just be sure you set it aside after you remove it and do not accidentally damage it by picking up just one end as it will bend under its own weight.
With the cover off, you will need to find the main fuse.   Just trace the black wire from the ac power cord until you get to the fuse.   On my oven, the fuse is a BUSS 20 amp fuse mounted in a gray fuse holder.   The fuse holder is easily seen from the right side (with the microwave oven facing you).   There is an area on the right side with a lot of wires and electronic components that you can see.   The gray fuse holder snaps onto a metal bracket and is easily (carefully) unsnapped with a small screwdriver.
Once you have removed the fuse, you can visually inspect it and/or test it with a continuity tester or volt-ohm meter.   Almost always, this fuse will be blown.   Replace it with a new fuse.
Now, before you put everything back together and put the microwave oven back in place.  plug the microwave into an ac outlet.   The LED display should light up.   The fan and oven light should also work.   Put a cup of water in the microwave and turn the microwave oven on for 5 seconds.   If it works, then you are probably "good to go" and can put the cover back on and remount your microwave onto the bracket.   Congratulations, as you have saved yourself at least $50! NOTE! One of the major causes of blowing the fuse is running the microwave without anything in it! If you stop the microwave to take food out, make sure that you CLEAR the remaining time left so that you do not inadvertently start the microwave empty.....or you will blow the fuse again!
However, if the fuse blows again as soon as you start the microwave, then you most likely have a problem with the power supply.   This will require professional repair.   However, the problem is likely with the power supply that powers the microwave portion of you oven.   In this case, you could replace the fuse and still use the convection portion of your microwave.
NOTE - do not attempt any of this unless you are very careful.   Make sure the the microwave is unplugged before you remove the cover and attempt to replace the fuse.   Don't run the oven without the cover on as you may be exposed to dangerous microwave radiation.   If you are inexperienced with this type of repair, let a professional do it! If your problem is anything other than the fuse, take it to an authorized repair facility!