fuel-tank Merc C-type
 

I am looking for technical information (and eventually, drawnings) about the fuel-tank of the Mercedes Type C diesel.*
As this type of mercedes has 2 fuel tanks, I like to have technical information about the system used to combine both tanks. *
Do they have separate measuring devices, or, is there only one measuring device combined with a equal-leveling system?*
I have the impression my proper C-type only use one fuel-tank. When the fuel-level on the dashboard shows 1/2, I am running out of fuel. And, I am only able to fill with about 30 liters of diesel. My problem is, do I use only one fuel-tank, or, is there something wrong with the measuring device?

The C-class appears to have 2 fuel tanks BUT there is only 1 as the 2 halves are joined above the driveshaft.
There are 2 sending units.
The fuel pick-up comes from one side & the return to the other side. Inside of the tank there is a passage that allows the return fuel to "pump" fuel back to the pick-up side.