HI Geoff,
For what its worth I'm a chemist who used to work for an oil company, and this is how I'd try to clean up your water tank (or get your vendor to do to for you). This assumes the tank is metal (see below).

Petrol/gasoline is a mixture of aliphatic (like iso-octane) and aromatic (like benzene) hydrocarbons. The latter are toxic, the former less so. Neither can be washed out using anything water-based as the stuff is immiscible - it separates at high quantities but forms tiny globules at ppm amounts (which is all you need to taste bad). Environmental agencies use a lot of detergents to clean out oil spills but these are more toxic in some cases than the hydrocarbon. Soda, bleach etc won't be helpful, as most of the hydrocarbon won't react with it.

However your petrol is volatile (lucky it's not diesel), so I'd completely empty the water tank, disconnect any pipes in or out, and blast air through the tank. We'd do this with nitrogen or argon ('sparging') in cases of severe contamination. You might connect up a leaf blower or vacuum cleaner. This treatment should cause any petrol to evaporate and be blown away - it'll probably clear out any other muck in the bottom of the tank as well.

What I then would do is to replace any plastic pipe or seals that the petrol might have come into contact with. Firstly the petrol could partly dissolve and weaken the plastic, secondly it could extract additives in the plastic which you would want to to drink, thirdly it could get absorbed by the plastic and seep out over a longer period, continuing to taint your water. If the whole tank is plastic then I'd say its a replacement job.

Hope this helps

KP