Hi Rich,
just for completeness (though I think it won't add new insights to the original question): with "RFC developer trace" that is mentioned above, they probably mean the one that you can activate in transactions SM50 and SM04?! In "SM50 -> Administration -> Trace -> Active Components" you select the component "Abap Proc." (which includes RFC) and a trace level like 2, and in SM04 you can then start the trace for a selected user session (so you have to first call RfcOpenConnection(), stop in the debugger right after that, switch to SAPGui/SM04, find the user session that got created during RfcOpenConnection(), activate the trace, switch back to the debugger and continue the C program...). Make sure to reset the trace when you are done...
The trace is written to the standard work process traces (so not into dev_rfc.trc). But I'm not sure, whether it will be helpful for "application level problems"?!
Best Regards, Ulrich