Is the trailer plugged into AC?
Is the Trailer wired for 30 amp or 50 amp service?
Is the water heater the fast turn around type?
The dual heat types have two switches that can be switched on.
One for gas one for electric then both for fast warm-up.
See if the air conditioner and the microwave come alive at the same time when your trailer is plugged in.
I'm just guessing here:
I'm thinking the micro and the water heater are on the same wiring loop.
50 amp trailers have two circuits running inside.
See if there's any other AC powered item not working i.e; the convertor for instance.