Careful to not crack your engine block dumping water on it this way.... Sudden cooling has been know to crack metal.
I assume the water level in radiator is correct.
First I would check the thermostat to see if it is working correctly. Best to just put a new one in by the time you go to all the trouble of taking old one out.
Then check water pump. Water pump might have a "weep hole" that will show leaks when the pump goes bad, bad pump means no coolant flow means overheating.
Do NOT go to an RV forum for engine issues, check out an engine/van forum that specializes on auto repair.
I know about RVs but do not know anything about your Mercedes 6 diesel, I know motors but not specifically your Mercedes 6 diesel.
How old is the van? Radiators will go bad, get clogged and this can cause overheating. Have you ever changed the rad water? Only issue if it has lots of miles, 50,000 or 100,000 is when this issue pops its head up...
