Modbus RTU requests should be processed the same way in the Data to Server as the Modbus TCP. To add the data to Data to Server JSON string, %a variable should be used. If you would like to send different data to different brokers, data filters can be applied. In the Modbus RTU case, Modbus Slave ID or request name should be the correct filters to use.
If you are still experiencing any issues, please describe how you configured the RTU slaves and attach a troubleshoot file to the original post. The file can be generated by navigating to System → Administration → Troubleshoot in the routers Web interface. This file will only be visible to Teltonika moderators.