Frank, the bladders are actually a dual purpose container when you get right down and think about it...
You're able to bring fresh water in from a remote location without breaking your boondocked camp down...Very convenient!...However, the old addage of "what goes in must come out" applies to when we all boondock, right?
After filling the fresh tank up, attach your maserator pump to the rv dump then attach the black water garden hose to the messy massy pump and bladder (hopefully relocated into bed from on top of Carl's pickup cab..Lol) and pump black tank holdings back into your previously emptied bladder to dump at a town sewer system of your choice...Very convenient!!
To everyone reading this, don't attempt to repeat this cycle of events more than once, please...throw your bladder away and buy a new one, K?!...Lol.