It can be confusing.
The toilet stuff always goes into the black holding tank. There is no way to bypass it. If you leave the valve open the liquids will escape the tank and leave the solids behind. Eventually the solids will build up and you will end up with a major clog.
By leaving valve closed until you are 3/4 full you'll have enough liquid in the tank to wash everything out when you open the valve.
Once the black tank is emptied,It's a good practice to fill the toilet with water several times and flush while the black tank valve is still open this will help clean out any residue and flush out the sewer hose. Or use a holding tank cleaning wand available at RV supply stores.
Also once you close the valve fill and flush the toilet a couple of times so you have some water in the tank to start with to help breakup the solids. The goal is to have lots of liquid vs. solids when you open the valve.
