Holding Black Water or Flowing Through

Thanks for everyone's comments. I do hold waste and dump together (as suggested) and do not leave the line open (as asked). My reason for asking was that after dumping, I got an odor in the cabin. Another suggested that I not hold, and that's when I posted looking for consensus.

The problem turned out to be that when I dump, the suction from the grey water pulls the water from the traps which allows the grey tank's odor to escape through the drains up into the cabin. The solution: refill the traps with water after dumping.

Thanks again!!!