Cleaning stainless steel water tanks is simple.
Fill the tank to about one-third - and drop in about half a dozen Steradent tablets (for a 45 litre tank).
Drive around for a couple of days - drain the water, give it a good rinse - hose in/tap on etc.
The tank is then clean.
If you have a stainless steel thermos that has been used for black tea or even white coffee/tea and it is stained - try a couple of the Steradent tablets in it - you will soon see why the above procedure works.
Hope this assists. I have a plastic jerry can in one of those metal holders in the back of my Defender - with a length of hose and tap on the end. Works well - and yes, every so often I clean it with the Steradent. No problems after 4 years.
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