Once you are hooked up to shore power, the microwave should work. If it doesn't, check circuit breakers in power panel. If the circuit breakers are all on, (turn off then reset on) the next thing to check is GFI outlets. In RVs, usually the first outlet on a circuit will be a GFI outlet with a 'Test' and 'Reset' button. The rest of the outlets are connected thru that GFI outlet, so if it's tripped, all the outlets in the chain go dead. Sometimes the GFI is outside in a compartment or even behind the microwave.
You should have a multitester in your toolbox to help find dead circuits and troubleshoot electrical problems.