I may stand corrected, but I "think" your 300E and my 300SEL have the same engine - M103 - single overhead cam - 6 cyl.
If I'm wrong, what follows may not work in your case.
Looking ay my engine from the front of the car, one sees 2
14mm bolt heads on top of the cyl. head just to the left of the engine oil disptick. The M103 engine manual coolant section shows a picture of this area. In this book, you're told to remove the 14mm bolt closest to the front of the car and refill through here. In other posts here and at one in particular that was answered today by a pro(Randy Durrance), it's recommended that you refill through the top radiator hose in order to push out trapped air. The approach offered in the 103 engine manual has served me well.
If you do not have this manual or the CD-Rom, send me your emailid and I'll email you the coolant section on the CD-Rom when I get home tonight.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.