It's all 'waste'....grey tanks can be as nasty as black tank---even worst
Rotting food, rancid greases/oils, decaying hair, sloughed body cells etc
Past two trailers (1980 and 2007 models) I have added an 'extra' dump valve on the common drain header. With it closed I combined both black/grey on the 1980 trailer and all three (black/grey/galley) on present 2007 trailer)
Funny thing...no issues with odors (black or grey) cause the 'vapor barriers' as designed worked and dumping the waste tanks when FULL allowed for that WHOOSH factor (fast moving/high level river moves boulders)
If only the forward black/grey tanks share common drain header and rear grey has separate dump valve/line (as indicated by only one black/grey valve handles in wet bay center) then the extra valve on the forward common drain header will allow you to combine the forward black/grey 'waste' tanks.
