Any idea what the little black box does? Is it wired to the incoming power supply? My first thought is that your power converter is sending out dirty power to the 12v but having the odd black box influence the noise is interesting.
Unfortunately, we recently got rid of our 94 or I would look behind the radio to compare.
I know in car audio that was usually an indication of something being grounded improper.
Current RV - 1999 Bounder 36s
Former trailers - '94 T-2770
'08 Keystone Sprinter 299BHSS