Wow! Thats a long trip in any car! I feel ya in the w126 seats. I have an '87 560 SEC and the seats are not great. Can't seem to get the right angle on the seats or steering wheel. My 300e is far more comfortable for any distance say 100+ miles. After driving to say, San Antonio which is about 2.5 hrs from me the 560 always hurts my lower back.
I also have a Volvo c70 Convertible which has probably the most comfy seats yet. But how did you feel about the car during and after the trip? Smooth ride quiet etc?
We have a 560sec which is elegant. An SL500 which is brutal performance wise, a 300E, solid, quiet and smooth and the Volvo, she's a rascal!