When I do complain about something, I try to figure out where to complain for it to be meaningful. So I try to short-circuit as many as possible and bring in as much facts/proof as possible to make it possible for the one with a saying to not need too much own work to forward the complaint the last steps.

So I would probably contact the main office directly, and supply photos showing a blank display.

As already noted, the staff on site don't have any say in things. And it would be extra work for them to try to forward any complaint. Especially since all they would only have the saying of one customer. No text to forward. No evidence showing the problem. Only evidence that they are selling fuel, i.e. that there isn't a show stopper.