Some Black Tank Question

Hello all,
          This will be my first timew every going camping in a trailer.  So sorry for the newb questions but
      Seeing the the person who did they walk through with us on our 183g was pretty much useless (Walk through person:  "Add some water to the tank" me "how much", person "some"
 
 So 30 gallon black tank, how much water do you add before use.  So I am heading out for camping do I add 1, 2, 3 gallons plus one of those tablets for waste (cause I assume you don't start with the tank dry) 
    And I also figure when I go to drain it  (I have to drain it every time as that is the rules at my storage facility)  So even though I have the black tank flush valve I am assuming I should still fill it 2/3 of the way then drain. So can I fill the tank with the flush valve or should I pour water in the toilet with a hose or if I have any water left in the fresh tank use that?
    And after I drain the tank and rinse it out can I store it empty, or should I add some fresh water and another table thing when the trailer is sitting doing nothing.

    I think that is all.  So thanks for any help on this.

Yes, put 2-3 full toilet bowls full of water down there, along with treatment of some type, after dumping.  Do not leave it empty or any remaining waste can solidify and cause problems.  Yes, make sure the black tank is 2/3 full before dumping even if it means adding water.