I don't understand the concern with having "clean" gray and black holding tanks. Like a septic system, I want my tanks to have a healthy living ecosystem of bacteria (the good kind) and enzymes. If one uses the proper amount of liquids when flushing, waits until there is 1/2 tank or more before dumping (or add water prior to dumping), and uses a quality digester (I use Rid X RV liquid), there should be no reason to clean waste tanks. In fact, I maintain that it is counter-productive.
Fresh tank, yes, sanitize with bleach once or twice a year.
"If the women don't find you handsome, they should at least find you handy." - Red Green