Actually there are different levels of working class. A guy earning $18k per year working in retail isn't exactly in the same boat as the manager of the store making $80k a year.
That was a story made up by capital owners to keep the working class fighting amongst itself. The reality is that anyone who works for a wage is being taken advantage of
The middle class are those who have the capital to be capitalists to some degree, but not enough capital that they can live entirely off that capital, still requiring some kind of job to make ends meet. Someone earning $18k per year at their job could be middle class and someone earning $80k per year at their job could be working class. Their job income doesn't tell us anything with respect to their capital holdings.
Again, middle class is a concept invented by capital owners to keep the working class fighting amongst themselves. If you depend on a wage to survive, you are a worker
Working class is defined as those who must sell their labor in order to survive. If you cannot live off income of your investments alone and must work for a salary then you are a worker, you are working class
If you can quit your job and live purely off your investments then you are a capital owner, if you cannot do that without adjusting your lifestyle you're working class