I am working on mine right now. There is 1 main 20 amp ceramic fuse inside the metal covers at the top right. If that blows all functions stop. See if there is a generic microwave or small appliance service center in your area. Most of them can work on these. The downside is that labor minimums are $50-$75 and the entire repair will come close to what an economy new one will cost. On the other hand if you get your old one repaired instead of replaced you won't be having to mess with adapting mounting and trim panels on a new one. Ventilation is a big question with a new one. You will see that the Apollo has a custom cooling exhaust fan blowing out to the top front (the factory fan blows out to the right bottom rear) added on by the RV microwave supplier. You should somehow mimic that for any microwave you install in the old space.
On Edit, 7/22/13: I replaced all of the microswitches ($35) as the fuse kept blowing as soon as I closed the door. That didn't solve the problems, so I got a Sharp conventional microwave for about $130 that has the same dimensions, modified the built-in support tray in the cabinet to fit the Sharp's foot outline and took the Apollo to electronic recycling. I usually bring my countertop convection oven/ broiler with me on trips, so I still have the convection capability.