Can you leave your RV sewer hose open when you're connected to hookups at a campground?

You can leave your gray tank open, but not your black tank (the one the poop goes in).

Why? The solids in the black tank tend to stick, while the liquids flow out. You want to empty the tank when it’s relatively full, because the volume and high quantity of liquid waste will help pull the solid waste out with it.

If you leave the tank open, the pee will flow right out, while the poop will stay behind and harden. Over time, it becomes nearly impossible to remove without professional help and will ruin your tank.

Technically, the same thing can happen in your gray tank, but the solids are often relatively small there (small particles of food and soap residue), so it generally doesn’t cause the same kind of problem.