Came across this on one of the forums and it sounds like an idea worth try. When we returned from extensive touring of South Australia our Thetford cassette had a heavy lining of sediment - a nightmare to remove mechanically.

"Every 6 to 8 weeks empty a packet of citric acid (often available in the baking section of the supermarket) into the empty cassette, 3/4 fill the cassette with water. Over a period of 5 to 6 hours turn the cassette over a couple of times. After this empty the cassette and rinse out. This removes all the scale build up and leaves the interior of the cassette very clean. I tried it out yesterday and as suggested, it did leave the cassette toilet very clean and no odour in the cassette at all."

On the same forum this was a follow up post.

"As well as using citric acid to clean out the Thetford toilet, we also use citric acid every couple of months in the following ways:

In our Lemair washing machine, 2 or 3 tablespoons in the bottom then fill with water and complete one cycle of the machine, clears all the soapy gunk out of the waste pipes.

Down the waste pipe of the shower, hand basin and kitchen sink - 2 or 3 tablespoons drizzled into the waste pipe, followed by a small amount of water - just enough to form a paste which will trickle down the waste hose, leave for a while, then slowly pour some boiling water down the waste drains, followed by a good flow of cold water. You will be amazed at the gunk that comes out of the pipes. "

We have not tried the Thetford idea yet so we cannot say if it will for instance have any effect on Bio Magic. We can't see it doing anything nasty to the Thetford chemicals.