Not sure about any of this since I don't understand 12volt systems well, even tho been messing with them for over 30yrs of RVing. But--I don't think the CB panel is hooked up as you have drawn it. No battery in system, no 12volt from the converter--I believe all 12volt power goes from converter to battery(s) then to CB panel, not to both at same time. Are you sure the converter sends power direct to the CB panel?
What do the battery(s) read when static and disconnected from the converter?
If you are correct with the converter reading at 14.3, then the problem isn't the converter.
Someone else with better knowledge will surely come along to comment.
Welcome to the forum.
Have you read thru the 12volt bible: http://www.marxrv.com/12volt/12volt.htm