OK. Responding to my own post. Right now I'm feeling pretty dumb, but I got it working. Here's the details if it helps anyone else. The last step (7) is the one that has me feeling dumb.
1. Use serial port Mode 3, character timeout.
2. RemoteRig serial setup has RTS/CTS ON (but not sure if this is essential)
3. Using 4800 baud rate. Will try a higher rate later.
4. Data bits, stop bits and parity to match the TNC (8, 1, no parity)
5. Null modem cable between the TNC and the RemoteRig terminal at radio RF deck side
6. Straight RS-232 cable and serial to USB dongle on front panel side to my laptop.
7. Turn the radio ON. Yes...even though RemoteRig was powered on both sides, you apparently have to turn the radio on via the front panel before the RemoteRig COM1 comes alive. Didn't see that in the documentation.
Radio: TM-V71A
TNC: Old TAPR TNC-2 clone with RS-232 interface.
Successfully ran straight AX-25 packet as well as sending a message via a Winlink 2000 2M gateway.
73
N3AE