RV toilets are not designed to have their waste exit lines left open continuously, even if you are hooked up to sewer.
If you leave the waste gate open, then liquid waste will run out while solid waste will build a "pyramid of poo" in the bottom of the tank, which can solidify and be difficult to get rid of.
The tank is designed to be kept with liquid in it, so that the solid waste floats around in it (and is ideally broken down by some kind of enzyme product you add to your tank), so that when you open the waste gate all the contents of the tank are flushed by the massive outflow of mostly-liquid contents.
Whenever it is time for me to dump my black water tank, I always prop open the toilet valve with a block of wood to allow the tank to become mostly full, and then I open the waste gate letting it all flush out.
Steve