Doug,
One thing is clear your "Raymarine expert" is not interested in solving your problem. You can take his/her number off your speed dial list.
I have a 15 year old C120 and have been running AIS traffic through the
NMEA 183 interface at 38,400 B for nearly 10 years. So the interface works, You must have some configuration error. I could try to walk you through a number of issues and still not solve your problem. Maybe someone else has
solved the same problem, but my recommendation is to take your issue to the raymarine forum (
Discussion Forum - Raymarine Technical Forum) where you are more likely to get a solution.
John