There are two types of holding tanks on any self-contained RV (not talking fresh water storage here). You have your Grey Water (sink, shower, dishwasher and washing machine) and Black Water (toilet).
When using your RV for an extended time and you are connected to a sewer system, most people will open the Grey Water tank and leave it open. Leaving this tank open will save you the time and energy of dumping the tank all of the time. You will use more water than you think, especially if you are doing laundry and taking showers in the rig rather than using the park or resort facilities.
You should never allow items down the sink or shower drain other than water or other liquids. It is best to avoid letting any food waste, or other foreign objects go down the drain. These items may sit at the bottom of the tank and cause issues including odors. If you choose to dispose of liquids other than water such as soda, you should turn on the water first and let it run during and for a minute or so after you have dumped the foreign liquid. This will both dilute the liquid and allow enough flow for everything to escape out of the tank.
If you find your Grey Water tank starts to smell or you have accidentally rinsed some items down the drain you may want to close the valve and fill the tank with fresh water, let it sit for a while then empty it. I have never had an odor from my Grey Water tank.
Your Black Water tank is a different story. This tank holds all of the waste from your toilet. You should always keep your Black Water tank closed except to empty it.
Most RV Toilets do not provide enough water to dispose of the waste each time you flush. If you keep your Black Water valve open, the waste deposits will build up in the bottom of the tank and cause issues from smell to clogs. Keeping the tank closed will let the liquids build up and allow the contents to escape when you open the valve. After you empty you may want to rinse the tank out and add a little fresh water.
The use of chemicals in your Black Water is up to you. Some people say you need them and some say you don’t. I personally use chemicals only some of the time, I found it a waste of money to use them every cycle. If you empty your tank often you will not get odors. Chemicals are mostly used to reduce odors and lubricate the seals. They can also help in the breakdown of waste, but if you are emptying often they may not make a difference. For me I find that using chemicals once a month keeps my tank fresh and working properly.
When it comes to the use of chemicals you should follow the instructions on the product bottle and the owners manuals of your RV. Choosing to not use chemicals is something only you can decide.