@malaki86 if you don't want to use a MySensors device for mqtt, I don't think you'll find a complete tutorial. But people have connected MySensors devices to nodered, and nodered can speak mqtt I think.
A possible strategy could be to
create a rs485 raspberry pi gateway https://www.mysensors.org/build/raspberry
Add MySensors nodes to the rs485 network
Connect the gateway to nodered
Connect nodered to mqtt
Alternatively, you can make the raspberry pi gateway use mqtt directly.