Charles_Gibbs:
As said, I would have liked to have had the Arduino do the switch as well, but if that can't managably be done with a bit simpler code, then I will live with what I have at this point.
I suspect you could use the Arduino to do the switching. Have you tried it in a co-ordinated way with whatever you need to do in the Victron program?
I presume you have confirmed that you can choose to view the input from either device on the Arduino Serial Monitor.
...R