It is typical for RVs to have a vent going through the roof from the black water holding tank to vent gasses without any kind of check valve. I am not sure if this is what you are describing or not.
And sewer smell is fairly common - not something unique to Thor. Not that I am defending Thor, but this is a very common issue.
There are several reasons for sewer smell. First, is owner education. Simply, some people do not use their systems correctly, nor are they told how to.
There are two kinds bacteria that break down waste; aerobic and anaerobic. Aerobic bacteria will only form in the presence of oxygen, and is relatively odor free. Anaerobic bacteria forms only when oxygen is not present, and is the stinky type of bacteria.
If there is oxygen present to the holding tank, anaerobic bacteria (thus smell) will not form.
In that regard, installing a check valve in the sewer vent is the worst thing you can do as it prevents exchange of oxygen in the holding tank... which would simply promote the production of stinky anaerobic bacteria.
Also (and this is more of a problem with boaters than RV'ers), never put anything in your holding tank such as vegetable oil to keep the toilet working. Some misguided advice was circulating for boat owners (their toilets have more complex mechanisms) that pouring vegetable oil in the toilet will lubricate the parts, but it also forms an "oil slick" in the holding tank which also prevents aerobic bacteria from forming.
Here are my tips to prevent smelly toilets.
1. always shut the ceiling vent fan off before flushing. If the fan is running, it simply pulls sewer gasses into the bathroom. I can about guarantee you will have a smelly bathroom if you leave the ceiling fan on when you flush.
2. never put anything in your toilet that has not gone through you - with the exception of RV type toilet paper. In my RV, Thor unwisely plumbed the bathroom sink into the black water tank.
This means I have to be careful not to put anything into the sink that might form an "oil slick" in the holding tank. So make sure you know your RV and see if any plumbing mistakes arise.
The fact that Thor has done this is evidence to me they don't totally understand the issue... but then again, I don't think this is unique to Thor.
3. use a tank treatment - I prefer the enzyme type myself.
4. periodically check your roof vent to ensure it is not clogged with leaves, spider webs, etc. You need oxygen flowing into the tank to keep odors down.
5. consider installing an upgraded vent such as a Siphon 360 (I did), as it creates negative pressure in the tank so there is always an exchange of oxygen, as well as pulling odors out of the tank through the roof.
You indicated you got the sewer smell on a windy day, it was probably due to positive pressure in the tank from the wind.
Again, most RV manufacturers use those same cheap ineffective vents that Thor does, so it is not really a Thor only problem. But a few manufacturers are finally going to the Siphon 360 vents as OEM equipment. They are about $35, but well worth it as they all but cure the problem. This is the number one thing I would add to my RV if I had sewer smell.
For the Siphon 360, the windier the day, the more negative pressure is created in the tank, and the more air is sucked out of the tank at the roof vent. Of course, each time you flush the toilet, the Siphon 360 forces fresh air to also sucked into the tank from the toilet, which is the direction you want it to go, and also refreshes the tank with oxygen, which promotes aerobic bacteria.
6. Make sure your toilet bowl seal is good, and keep a "water-plug" in the toilet. A "water-plug" is a inch of so of standing water above the toilet drain valve. Like the trap in your house toilet, it prevents gasses from seeping up from the holding tank.
You create the water plug by holding the water valve down 1/2 way after flushing (in most toilets) which fills but does not drain the bowl. You don't need so much water that it sloshes out of the tank when driving down the road, but enough to create a trap.
7. throw a couple of Efferdent tablets into the toilet bowl (and don't flush, let them dissolve in the water plug) when not using it for a few days.
I have to say that I do not have any sewer smell in my RV at all, but I do the things I mentioned above to prevent it from occurring. This is common advice for any brand RV or even boats (which are even harder to keep odors down).
Here is a photo of the Siphon 360 tank vent I installed on my RV.
If your vent looks like the one below... that is probably the chief problem:
This is the vent 90% of RV manufacturers use.
The reason I am up on this is; I used to have a lot of problems with my boat's system, until I read Peggie Hall's books; she is known as the "head mistress" in the boating world (toilets are called heads in boats). She is very knowledgeable, and is where I learned about aerobic vs. anaerobic bacteria, etc. Peggie is also a consultant to the boating industry.
P.S. I have no affiliation with the Siphon 360 - I just am a RV'er that installed one, and found that it works.
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