You can generally leave the gray water open and drain continiously. The black tank will need to be kept closed and filled to at least 3/4 full before you dump it. Tis allows the solids to liquify and also the volume helps flush the tank. Also keep some potty chemicals in the black tank.
If you leave the black tank open, you will have the solids accumulate and the toilet is open to the sewer line smell as well. Some campgrounds have some back pressure and smell on the lines at times and can get back into the camper, so you may need to keep the gray tank closed as well.
I use the flush rig on our black tank about every 3 or 4 dumps.
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