Hi David - the sketch for sending TMCC commands to Base is the "TMCC Snippets" sketch (and the ByteBuffer library) attached as files to this post upthread.
The sketch for reading commands from the Base (serial echo) is just what I've pasted into this post upthread. You can just copy that text, start a new blank sketch from the Arduino IDE, and paste the text into the new blank sketch.
I think I've missed where you posted the output from the echo sketch. When you run the echo sketch, do you see the "echo starting" message, but then absolutely nothing else when you fiddle with the remote?
If so, it's time to start low-level hardware tests. I am still wondering about the cable between the Base and your RS232-TTL converter. In any case, let's start probing voltages. Can you post some close-up pictures of your converter, and its connection to the Mega?