You can't expect to mistreat the tanks then dump some magic elixir in there to solve the problem. Per above, the key is lots of water and repeated flushes when the situation permits. Sometimes you have to dump and go but doing a rinse cycle or two when you can will get out a lot of extra crud.
Also, I'm a believer in driving from place to place with the black and gray a quarter full, with a handful of borax and a squirt of Dawn in the black tank, and a glug of Zep citrus cleaner in the gray tank. Dump when you get close to the stopping point and you have clean empty tanks ready to go. It's not a magic bullet but as an ongoing process I think it's a valid method. Just doesn't make sense to deal with chemicals and smells all the time when water can take care of it.
Found a youtube video that describes this very process, so no point in restating everything:
https://www.youtube.com/watch?v=wUWGlm2gQfE
Mark B.
Albuquerque, NM