For those who are not familiar, there is a German company called SOG that makes a retrofit for Thetford cassette toilets. My installation was for an XP Camper V1, but I thought this would be useful for anyone else who has a Thetford Cassette toilet. I highly recommend this upgrade. No chemical or other smells from the toilet. It's actually pretty remarkable how well this system works. The SOG II comes in a "floor exhaust" version rather than the original which required you to install a hatch in the camper.

I ordered this unit directly from the manufacturer in Germany and had it shipped to a friend's address in Barcelona while we were there in October. But, they can also ship to the US. I speak German, so ordering was easy, but the guy who picks up the phone at SOG speaks good enough English that you can order. This unit is the "Type D" for Thetford cassettes. http://www.sog-systeme.de/?changelang=2

Basically, the SOG II system utilizes the vent in the Thetford cassette. You attached an exhaust hose to that vent opening, and every time you open the toilet blade, a tiny fan creates suction in the cassette and pulls air through the exhaust hose where it passes through a charcoal filter before being expelled to the exterior. Even though it's designed to vent through the floor, I vented mine through the front wall of the camper near the water inlet. Aside from the obvious benefit of pulling smelly air out of the cassette when the blade is open, SOG claims that the air movement creates an oxygen rich environment around the waste in the cassette which speeds up the breakdown of solids without the need for chemicals. Once your toilet is chemical free, you should be able to dump it at any Park Service pit toilet, or conceivably, dig a deep hole and bury it.

In my opinion, this unit should have come standard directly from Thetford and they missed an opportunity by not developing it. SOG does not have a US distributor, so you are on your own for future servicing. I ordered several spare charcoal filters with mine since those are meant to be replaced every year or so and I figured it makes sense to ship everything all at once. We originally had started using our toilet with the chemicals, so it took several weeks for the chemical odor to dissipate where it had absorbed into the plastic, but now there is no chemical smell. Your mileage may vary, but for us, this has made the toilet as odor free as I could ever hope for in a small, confined space. While snow camping, we didn't have the windows open, yet we could use the toilet with no smells, provided the bathroom exhaust fan was turned on. Perhaps more importantly, the slight background smell of pit toilet chemicals is no longer present in the camper. That smell is not overly offensive, but I didn't like having it in the background, so the SOG is worth the cost and effort of installation for me. We use regular Costco toilet paper instead of the special RV stuff, and have found that the toilet paper and solid waste breaks down quickly in the cassette despite the lack of chemicals. The unit is not very expensive. I can't remember exactly, but it's something like $200.

The installation was easy in the XP Camper. Other campers may require different steps, but this will give you the idea:

1) drill a hole through the "base" of the cassette housing to expose the vacant cavity above the gray water tank.
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2) Install the switch on the underside of the blade mechanism. This is easy as long as you think through it and carefully test fit several times before you permanently stick it in there. Make sure you have conceptualized how the blade mechanism works and how the switch works since this is a switch that is "on" when the blade is open. That means it has to be positioned correctly or the fan will not turn on when you open the blade.
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3) Fish the switch power lead through the exterior of the cassette housing and tap it into the power wires for the toilet flusher.
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4) The kit does not come with a 90* elbow so you have to make your own for the point where the exhaust hose attaches to the base of the cassette housing. This elbow is necessary because the clearance between the top of the gray water tank and the bottom of the cassette housing is a little tight. I found that a 3/4" copper fitting exactly fits the SOG exhaust hose and that a 1/2" male threaded fitting can be threaded into the SOG adaptor, so I used a brass street elbow and it worked perfectly (this will make more sense once you have the SOG kit in your hands).
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5) Route the exhaust hose wherever you plan to install your charcoal filter. I made a simple bracket out of aluminum angle stock that uses a 4" hose clamp to secure the charcoal filter to the top of the water filter bracket which is located under the small triangular hatch inside the camper. The tiny fan motor plugs into the charcoal filter.
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6) Drill a hole in the side of the camper for the exhaust. I decided to route mine out through the area next to the water fill. The kit does not include enough hose for this, so I purchased the white hose visible in the picture of the filter to make the connection to the exterior.
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7) Enjoy a cold beer. Your're done.