Your task is to define a RationalNumber class in Python. This class should be used to represent a rational number, a number that can be expressed as the quotient of two integers (numerator and denominator).
The RationalNumber class should have the following characteristics:
The class should have two private integer attributes, the numerator and the denominator of the rational number.
The class should define the magic methods __add__, __sub__, __mul__, and __truediv__ to add, subtract, multiply, and divide two rational numbers, respectively.
Addition, subtraction, multiplication, and division operations should be performed in a way that's mathematically accurate for rational numbers, with the result being another instance of RationalNumber.
The class should reduce the rational number to its simplest form by dividing the numerator and denominator by their greatest common divisor (GCD).
The class should implement the __str__ magic method that should return a string <numerator> / <denominator> in its simplest form.