Acrid Chemical smell from Toilet / Black tank in 2016 320S

If checking the P traps does not solve the issue, I think I would flush the thank a few times even though the water is clear, you might have some powder or other caked somewhere.

@LeeM use a full scoop of Happy Campers and between that and water in your P traps I bet your problem will go away. I used a half scoop at first and now use a full scoop. I also put a half scoop in each drain with water for the grey tank. Since it’s enzymes it’s not a bad idea. Good luck!

@LeeM, it is recommended that you fill your black tank  full before dumping.  The idea is to prevent anything from clinging to the sides.  So, if only partially full, add water before dumping.  Along those lines, if hooked up to a sewer, don’t leave the tank valves open - only open them when ready to dump.  We use Happy Camper, too, but consider adding Calgon or Borax to both tanks to keep the inside of the tank slick which prevents cling.  Google “Geo Method”.

I always add about 2x the amount of wheater if what I am flushing. Insufficient liquid leads to build up.

use just a bit dishwasher detergent together in the black tank,    our poop does have lots of oil and it clings to the tank the detergent seem to help 

Sounds like you've got a plan! I usually add Dawn dish soap, too. I don't think it will hurt, and it probably helps! Have fun at UCamp! those of us who can't come are grateful for pics and stories! 

We scrape our dishes and wipe them with paper towel before washing in the sink, but sometimes end up with a solid white greasy substance when we drain the gray water... Have been adding lots of hot water with Dawn to cut the grease and that seems to help.

I had a similar experience during 2 month work camping last summer. Initially I was sure the smell was coming from the black tank, although it didn't smell like a sewage, but by the end of my 2 month camp stay I discovered the smell was coming from the gray tank through the shower drain. I was emptying the gray tank regularly so I couldn't understand why there was an odor. Towards the end of the camping I also noticed that the gray tank level indicator stopped working. I discovered the problem was not being able to power flush the gray tank and during the long stay, the soap scum and food particles stuck to the tank. Before the 5 hour drive home I had filled the gray tank half way with water, added little bit of chlorine and dish detergent. When I got home I emptied the gray tank and noticed the water coming out was gray and full of sediment. The level indicator works again and no more smell. I would start with the gray tank.

Denny16 said:

When the trailer is connected to city water, the water lines in the TaB are pressurized from the city water connection, not yiur fresh water holding tank.  Just connect to City Water connection, turn off the pump and run water from the sink and shower to get the gray tank filled to the level you want for cleaning.  Your fresh water tank and pump are not used if you use the City Water connection.  We give our water tanks a good cleaning this way at the start and end of each camping season.
cheers

Yes, got that. With city water I can estimate gallons per minute (gpm) coming out the sink faucet by collecting it in 32 oz. measuring cup and a stopwatch. That will get me the 18 gallons I'd like to get in there for a decent cleaning. I guess with black tank I can keep flushing toilet 'til the monitor panel shows it 2/3 full. Should I let the Happy Campers product soak for a couple of days?

For the black tank, if you do not have a black tank flush inlet, you can put a garden hose in the toilet, and see the water level rise by holding the flush valve open with your foot.  You will see when it is full.  Joh, your 2020 TaB 400 has the black tank flush inlet, which will fill the black tank very quickly.
cheers

Is there a p trap under the shower drain in a T@B 320S. It doesn't seem like there is enough vertical clearance for a p trap.

You need to find the source in order to fix the issue. Poor an inch or two of water in the toilet bowl and leave it alone overnight. If the water is still there, the toilet seal is good. If the water is gone, the seal is leaking (in both directions). Sometimes cleaning the seal will correct this, sometimes you need a new seal.

While we are on the toilet, we use Digest-it. It is outstanding. The Thetford brand is equally good plus it contains a deodorant and a mild perfume. Camco sells a "toilet cleaning wand". It is a stick with a small sprinkler on the end. We use ours once a year. I have heard RV maintenance people discussing the benefit of leaving a gallon of treated water in the black tank during storage. They this it keeps any material left after dumping from drying to the bottom of the tank.

If the toilet seal is good, the problem is either the sink or the shower drain. The lavatory sink has a one-way valve, not a p-trap. Many 320 showers have a valve also. Over time crud gets stuck in the valve. Bacteria and fungus grow in the crud and prevent the valve from fully closing. This produces an odd odor from the growth plus gray tank smells. You need to periodically clean the valves. We don't use our inside shower so we need to clean it more often. As the gray tank fills, it backs up to the shower valve.

I'm going to make a contradiction. Be very careful putting any cleaning agents not specifically designed for RV use down any drain. The trailer is not built like a house. When we get the smells you describe or the gray dumping gets slow, it is time to clean. A company called Roebic produces a "Bacterial Drain & Trap Cleaner." Put it down the drains a few days before the next outing. Drain after camping. It works really well clearing both problems; contaminated Hepo valves and the grease/soap buildup in the gray tank drain. Also, your tank sensors are accurate again.