The environment inside the fuel tank sounds pretty bad when considering saturated petrol vapour. In tank electric pumps are common these days. What sort of plastic should the wire be made from? What fuel is worse? Petrol? Diesel? Alchohol? Sure, plactic fuel cans exist and even plastic fuel tanks, like on a ford focus, but what about the internal wire?
Those are mostly fluoropolymers (Viton, ETFE to name two).
As with most things that are fluorinated, those cables are unaffected by most chemicals (good), expensive, biopersistant (not good for the environment) and produce hot hydrofluoric acid (especially bad) when burnt.
The search term you need is "cable chemical resistance chart"
A nice one: