A friend told me he met a couple in the local caravan shop who were returning from holiday over the alps.They had emptied the loo and to save time in case they needed it on route,refilled it with chemical and put one pint of water in it then shut the blade over.It was when nature called they discovered the loo had exploded.They quiered this and were told that it was the high altitude that caused this.
The older models used the blade itself to open and seal the vent plus they were very prone to "gung-up" so its understandable in that case. These are best left "just closed" but not as tight as they need to be for removal and carrying.
But with modern ones (C200 etc), the vent valve is automatically opened on inserting the cassette. If that valve was serviced every year then its not going to happen. Its easy to twist the valve assembly and remove for cleaning with an old, tooth brush; remember your "marigolds".
I think you'll find that it has nothing to do with the chemicals but the fact that the cassette was sealed at low altitude and then taken to high altitude where the air pressure would be lower causing the cassette to split (bit like blowing it up like a tyre).
Leaving the blade open when traveling between high and low altitudes will prevent this
Check with Thetford but I think you'll find that the blade does not open on insertion - only the top cover - ours, a C250, certainly doesn't
------------- Caravanning is a way of getting a cheap holiday out of an expensive hobby
i read an article about this and how a couple tested this theory and worked a treat, ka-boom.
i wanted to visit some high parts of europe and now will more vigilant on the altitude