I use the Blue Streak system which puts a little chemical in with each flush --- causes blue water. I have had no odors at all with this system and it also has a chemical in it which lubricates the dump valve. Like others have commented, lots of water to flush when I dump and I also add a little Calgon with each dump. Helps to keep the sides of the black tank lubricated for the next dump.
Dave and Karen -- '02 Ultimate Advantage 36 C, 350 HP Cummins, Allison 3060 Tranny
'07 Ford Escort, Brake Master 4160, '13 Ford Edge, InvisiBrake