We have a grey tank flush on our present 5er. We also have a dishwasher and I heard/read somewhere that that is the reason. I guess there are folks that would not bother wiping them of excess food stuffs before tossing them in the dishwasher.
Our previous 5er didn't have the grey tank flush, but as fulltimers and often staionary for months I would before making a move close the grey tank and run the waterheater till cold along with several tablespoons of DAWN. Dumping at our next stop and using a clear elbow alot of yuck flushed from the grey.
Since we don't use the dishwasher I'll just keep using my old method of flushing the grey.