Each need is different, but my take is that in a pinch you could purge your grey tank in some cases without issue. Black tank is a whole 'nuther story.
If push came to shove, I might get one of those portible tanks and empty my grey tank that way if I was in a situation where the grey was getting full and I couldn't purge it on the spot. It could also be used to empty the black tank too. I know many folks that had the 28 footer that had the combined tank a few years back....few of them liked it and Airstream eventually switched back to seperate tanks FWIW....
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91