We had a 2350 on the Ford chasis, so I'm not familiar with any difference with the Sprinter. If you don't have water anywhere under the chasis, all your drain lines are apparently closed. The only thing I can think of that could cause your problem is that there is a syphon action taking place when you're moving - and the water is coming out your overflow line. Don't know how that would happen, so I would suggest you call the factory. By any chance are either of your holding tanks filling? But, if you don't have your water pump on, no water should be leaving the fresh water tank. Very strange...