And I believe, it's either port 3 or port 4 ? And is Baud rate correct. Also, was your wiring harness built with the shielded data cable that goes from the box to GTR225 ? Also make sure the baud rate is set correctly.
I know, having an early RV-12 and box, for my SL-40, I had to buy an additional separate shielded cable from Van's to get that information to show up at the top of my Map, as it was not included in the original wiring harness, but was subsequently added later on, in production. You could probably make your own cable, later on, folks said the shielding wasn't necessary, just need the pins, and pinout locations.
Per previous reply to Tony, radio set up on serial port 4. Radio was supplied with the mounting tray and harness by Vans, admittedly some time ago. I suppose it is possible harness has been modified through experience, will also contact Vans.