At that speed, I would be pleased with your milage. The trip sounds like fun, but I don't think you could expect more efficiency from the M104. I've never run my 1997 C230 (M111) at that speed for that long, but at 75ish, I get about 35 mpg. At 75ish, my 1995 E320 Wagon (M104) is in the middle twenties. Good luck. Have fun.
1988 300 SL (5 Speed)
1994 E320 Wagon