There is also the possibility of algae contamination. Who knows where the tank has been filled!
The ideal thing to my mind would be to get it steamcleaned, if possible.
First thing I'd do is to empty the tank, fill the thing with a mild solution of Sugar-Soap, remove the solution next day (to use to wash the van with) then stick the hose into it to the bottom, and let it run. Petrol floats, but it also sticks, somewhat.

I have an vague idea that petrol is not especially poisonous, but I'd check, in your place.
There may even be a trapdoor on the top of the tank, for problems such as yours, or indeed, you could make one with a piece of plastic/fibreglass and drivescrews. Arranged with a gasket where it could only leak when the tank was absolutely full.

Brush manufacturers can make bottle-brushes of any size with stalks as long as you specify, for the pipes.
Then you only have the pump to clean. :-)