Thanks for the help, editing the systemd file did the trick. The change that worked was changing the ExecStart line in /etc/systemd/system/rigctl.service
ExecStart=/usr/bin/rigctld -m 1020 -r /dev/ttyUSB0 -s 38400 -P RTS
38400 is the baud rate my radio is set to.
I had been running the 'rigctl' command to experiment with different flags but I think this was interfering with the systemd service already running so it wasn't working right. Maybe stopping the service first would have worked better.
Drew
K4CYB