As Johnhef mentioned get the battery tested first. I've seen units with a defective cell that appears to take a charge but then that cell drops to zero volts under a load. You'll also see the voltage slowly dropping after removing the charger.

If the battery checks out fine then the most likely cause is corrosion in a wiring connection. Remove the cables (especially where the ground is connected), clean everything with a wire brush, and use sealant spray or electrical grease when reattaching.

If it is a bad battery and the store does not have an appropriate replacement you should note that MB roadside assitance will deliver and install a new battery for about a $60 premium over the battery cost in most places.