Mercedes tool # 202 589 00 90 00. cost is ?? You can order one from the dealer. Have you called the dealer to see what the charge would be for them to drain the tank. The cost of them draining the tank may be the same price as you buying the tool. Maybee you could find the tool on E-bay and then when you are done with it put it back on E-bay. Although if you put the valve back into the fuel rail on the engine and connect the correct type of hose, refill the tank, the tank should drain correctly. But that may be a waste of gas. Go to this web site
www.mercedestechstore.com
select- Access our Training Print Files
go down to the heading - Files for 508 Systems 2 training
select - 508 Systems 2/508 HO Part 07 Fuel supply (WJB) 04-01-01.pdf
Note: you have a Model 208, this will discribe your saddle tank and the operation.