This sounds like my Alternator failure - only mine was catastrophic, i had to change the alternator, right away, in my driveway.
(NOT to say other suggestions are wrong...)

By taking the car to the dealer, you have set yourself up for a financial disaster. Go get your car and take it to a local shop known for Mercedes expertise.
(Unless you took it to a Mercedes dealership, in which case you are very fortunate to have a dealer that will work on it- tho I'd still avoid even a Mercedes dealership.)