I think there's a drain plug in the bottom of the tank that houses the fuel strainer. You'll need a 22mm hex/allen key to remove it. They are available on ebay.
The W202's are fitted with two saddle tanks connected by a bridge over the prop-shaft, you would need to undo both drain points (assuming there are two) to empty each tank.
Alternatively pull off the fuel feed pop it into a tank then switch on and let the lift pump do the work
Give the pump a rest after a few minutes before carrying on.
A more enjoyable way is to find a nice empty motorway, assume a speed in excess of 100MPH - when the speed decreases considerably the tank should be empty.....