What is the value of lcd_key after it is called?
lcd_key = read_LCD_buttons();
Does it equal btnHITCH, btnLEVEL, etc, or does lcd_key = 1,2,3 etc?
Some print statements as Morris suggested will help:
lcd_key = read_LCD_buttons(); // read the buttons
Serial.print (lcd_key); // what'd you get back?