2001 clk 55 amg fuel sender/fuel gauge problems

Following a major engine service at the dealer my gas gauge will no longer read full. It reads just under 1/2 even when filled to the top. I have purchased new left and right senders but am uncertain of their location,  and how to get at them. I  think they are removed/replaced from under the car, behind a cover with three bolts with the tank in place. Is is necessary to fully drain the tank in order to replace them? If tank must be drained what is the best/easiest way. Also I had the fuel pump replaced 7 months ago. Since I believe this car has a saddle type fuel tank I wonder how fuel gets from one side to the other if the sender with the pump is the defective one. In other words, when gauge reads less than 1/2, but car wont accept more fuel , are both halves of tank filled?

Thanks in advance for any help