Davydd's got it pretty much covered.
It never hurts to top off your holding tanks with water/treatment stuff,
and dump them, even though you know they may not be completely full, as Davydd
suggested. Just before departing a campground, or at a fuel/dump station or truck stop.
Empty tanks = less overall weight = better fuel economy, for one. More capacity available
when needed for use, for two.
Some folks suggest it's a good idea to use a little coconut oil to keep the valves lubricated,
or I believe RV antifreeze is also good for that. It also is supposed to help keep your tank
level indicator sensors cleaner, too. You may have heard of putting "ice cubes in the toilet"
and then flushed into the tank to loosen black tank "debris" prior to dumping. You dump a
bag of ice cubes into the toilet and flush them, then drive around for a while before dumping,
to get a more thorough "scrubbing" out.We also only use our toilet for liquid waste,
so we don't do the ice cube thing.
One thing we found out (the hard way) when filling the gray tank with water
by filling our galley sink with a hose, is make sure the low point shower drain
plug is secured. In a Roadtrek with aisle shower, there's a floor drain hole with
a plug which blocks back or overflow when the gray tank is full or filling. First
time we filled our tanks to practice dumping, we didn't notice the previous owners
left it in place, but loosely, as they never used the shower, and rarely used the sink.