Suggest that you carefully look for signs of a head gasket leak. This just happened on my 300E.
Coolant was disappearing but no signs of a leak.
Turns out it was leaking into the front cylinder. Pulled the plugs and found one wet one. Cranked engine with all plugs removed and water came out like a geyser from the front cylinder.
How is the engine temp? If it starts to rise above normal, turn on the heater and pull over to let it cool.
Call a flatbed or you might warp the head(s).
Coolant should not leak. It's going somewhere and white smoke usually means a head gasket leak.