I was working with the very helpful folks in chat, and in the process of troubleshooting the missing RRD file, I had changed the parser from 'Auto' to 'Nagios'. Changing the value back to 'Auto' does not appear to have an impact.
zencommand debugging did verify that the command was pulling the data correctly. The problem was the name of the data point. I tweaked the output from OK|total=$NUM to OK|Amperes=$NUM and the RRD was created and we're good to go!