obee replied:

The water board puts chlorine in the water and it disappears over time. Put a good slug of the stuff in now and empty the tank when next you go out. Household bleach is a weak blend of chlorine and will do the trick.

(oh yeah dont forget to put fresh water back in.

Actually the water board wants you to run your taps when you get home from a period away so chlorinated water replaces the old stuff in the pipes. Dunno if anyone ever does it.

Owen

lifeisgood replied:

I dont bother too much anymore with the purity of our van tanks.
We carry easy to clean 2o litre plastic tanks for good water and top up the van tank before leaving or en-route with whatever reasonable water we can get. Some areas its not that flash. eg bore water. We only use the van tank for washing up etc or boil it if drinking it (coffee ) and fresh is getting low.
you also have a little insurance in case you rip the hoses off your van tank and lose the precious fluid. (particularily if remote camping and not near a good stream)

Wizard1 replied:

When we get home I use the left over water to wash the van down by attaching a hose to the drain outlet and a Gerni.

If left in too long it will taint with the flavour of plastic so I may as well use it up. Otherwise I would only drain it onto the ground.