Like the title says, loosing fresh water out of the vent behind the axles. We have an 06 Landmark Mt. Rushmore. While set up and sitting level, I fill the fresh water tank till it overflows out the back. Check tank gauge, yep its full. Hook up and head out to our first stop, which is a dry camp stop. We arrive and setup, check the water level, we have lost about half of our water in a 5 hour trip. With some investigateing, I found it is running out the overflow/vent while going down the road. I mentioned this to the dealer and his response was, I shouldn't be traveling with a full tank of water anyways. Said I would crack the tank. Right or wrong, I installed a valve in the vent. After filling, I close it. I don't loose any water now. My question is, has anyone else experienced this, and if so, what did you do to fix it??