It could be a drain valve. The long tank is actually a water tank to store boiler water for a steam generator. However, the unit may have been rebuilt with the steam generator removed but the water tank left.
The fuel tank is the thing on the right in FRONT of the air tanks, not between them (note the sight glass at the top, right of the tank).
Dave H. Painted side goes up. My website : wnbranch.com
If it was the Fuel tank I would assume its the condensation/ crud drain.
Actually that is the water tank, the fuel tank is the thing to the right.