Hello, I am having the same problem. How did you manage it by hand in firmware without an interrupt? Did you have to poll to see when frame was complete?
I didn't find any real good solution, if you can't manage RTS by UART functionality, you can start a timer with the amount of time left for the last byte, or poll the TEMP flags. You can also, (if the driver receiver is always enable in RS485 case) waiting for receiving emitted bytes, it's not a nice solution but it should work.