Here is a pic of my first ever bike. The Suzuki Across (GSX-250)
It had a 12L fuel tank that ran under the seat and the filler cap was at the rear above the license plate. The 'tank' was actually a pop open compartment that you could store your helmet in.
You can also fit half a carton of beer, a bottle of port and a bag of ice in there
or so I'm told ....