It's considered best practice to allow your black tank to fill up and periodically dump it as opposed to leaving the valve open to the septic/sewer as you would a house. By allowing it to fill, liquid waste and flush water with assistance from gravity (and a black water flush) will help to push the solid waste contents out of the tank. By leaving the valve open, the liquid waste and flush water will slowly trickle out and the solid waste (including toilet paper) will tend to accumulate in the tank - particularly at the outlet.