I know of no logical reason to refer to certain items in the feminine, other than a man's propensity to refer to his possessions as he might refer to his wife. In your specific example, I believe that "fill 'er up" is more or less a fixed phrase. Other native speakers may feel differently about all this.
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jose gongoraThank you for your answer. That reason that hadn't come to my mind. Yes, that might be it. But, I sort of feel that there is something else behind it. Anyway, maybe you didn't see or it was just a too stupid question; can it happen vice versa: can we call an object by "him"?