Microwave fitting and powering.
I fitted a small microwave in my conversion. Basically the cabinet was built relatively tight, and then the oven was secured to prevent lateral movement with wooden battens. An additional metal strap was used to ensure that the oven could not bounce up off the base of the unit. I have travelled circa 7000 miles since the fitting, without any problem. As for power, I have provided a 4 KW inverter which peaks at 8 KW, and is only used when the engine is running. The power is provided by the vehicle's alternator, via a battery to battery charger from Sterling Power. The same power source is used to power my better half's 2 Kw hair dryer. Both of these appliances are only generally used for relatively short periods, so running the engine for a few minutes is not a problem. On a slightly different note, I heat up my water using 240 volts whilst driving, pulling circa 800 watts.
P.S. The microwave is only a secondary or occasional cooking source. My main cooking facility is a diesel powered Wallas unit.