I am trying to drain the tank on my dad's 1984 240 for him and all I can find on the bottom of what I think is the gas tank is a Phillips head screw. It is towards the center of the pan but slightly on the driver's side is that the drain plug

 

never heard of a drain plug on a gas tank on a volvo.

when I've needed to get rid of bad gas, I've always used a siphon pump from the top