I agree with Tanksowner.

However, if that doesn't do it (i.e. alternator check out ok), then the next place would be to look for a draw. A few years ago our 300sd kept losing charge despite a new battery & good alternator. In the end, I chased down a draw from the power antenna. It had enough juice to turn on dome lights/exterior lights etc, but never enough for cranking the motor. BTW, if this is the case, chasing down a draw is fairly straight forward (even for the less electrically inclined people like myself). Keep us posted.