How to clean water tank

Clint,
Try using Bi-Carb or Vinegar. Often hard to feed into filler, but I use watering can. Try to get up to about 10litres into tank and take for a drive if possible to mix around and you may have to repeat. I usually store my tanks full (less air for the nastys to grow. Make sure you run mixture through all taps. Hope this helps!
Regards Zappa

Clint Hi,
I use sodium metabisulphide available from Home Brew shops or HB sect of major supermarkets.
Mike.

We had a nasty bug in our water tanks and was advised to add a cup full of plain household bleach to each tank,fill to the brim, let each tap run a little while with the waterpump till you can smell the bleach, then top up tanks again, leave for 24 hours and flush out with fresh water.

We did this and have had no problems since. Its cheap and effective.

Basically its just chlorine.

Clint,

Do you have a problem with the water in the tank or is it just a perception because it has been sitting for a while?

If the water doesn't have "bad taste" or is not "smelly" then you may not have a problem at all.

Try simply draining and refilling the tank. If you strongly believe you have tainted water, then Milton baby bottle wash is all I have used. The quantity to use can be calculated from the label on the bottle.
Drain the tank fully, add the quantity of Miltons and refill the tank. Allow to sit overnight, then drain, fill with clean water, drain again, then fill up the tank.
It also pays to have a length of "food grade" water hose to use during the filling process so no "plastic taste from degraded garden hose is added to the water supply".

Apart from an obvoiusly tainted water problem, the water should be capable of sitting in the tank indefinitely, without problems.
Just check you don' have any clear plastic water line which allows light to enter and help generate mould in the lines. Black plastic water line is best where exposed to sunlight.

I always top up the tank after returning from a trip so there is little air in the system. It may be months before I use the camper again, but I have not experienced tainted water in my camper's system.

As afore said, unless the water tastes or looks off, no need to do anything. There are several threads on the same topic if you can find them with a search. Our water can stay for years between and being inside dark containers, is in better condition than in our household rainwater tank. If there is a problem (eg you have picked up unpleasant tasting water at some stage), i recommend a weak solution of household bleach. Those who have swimming pool chlorine find this a cheap option. Half fill the tanks - go for a drive so it sloshes around a bit, pump out then pump a bit more water through. All should be clean and fresh.

Motherhen