you should not have to ground anything with an isolator.. thats what you are trying to avoid is the grounding of something
BUT depending on how its isolating you may have issues.. as you mentioned
a good isolator will have transformers in it, cheaper ones do it with electronics, and usually work ok
real cheap one will just lift the earth / ground and you will only get the + wire fed thru the device..
you mentioned that with a short rca cable you got no buzz.
check and re check where your "long" cable goes, past or along with ANY power wires, video cables etc?
did you make your own rca cables or buy pre made? do they have any shielding? and is the shielding the wire that carries the audio... or is there a red and black wire and a metal shield?