Using STM32F10x RTC to trigger some function
You should use one of the hardware timers for this, not a RTC.  The RTC is useful for keeping track of the actual time-of-day and the calendar day (absolute time).  But you are just trying to count a relative time period of 20 seconds.

Configure and enable the hardware timer so that it provides an interrupt every 10 milliseconds.  (Read the datasheet or find examples of how to do this.)  In the timer's ISR, increment a counter so that the counter value represents the number of 10 ms periods that have occurred since the counter was reset.  Draw the four digits from the counter value on the display and draw a decimal point between the middle digits.  There's many different ways to do this.  For example you could use sprintf to convert the counter value into a string and then call LCD_DrawString.  Or you could repeatedly divide the counter value by 10 to extract a single digit and then call drawchar for each digit.