It is coming out of the tank vent. You don't mention what rig you have, ours is a Landmark, we had the same issue. Would loose about a 1/3 to 1/2 of the fresh water in transit. I did some investigating and found the tank has 2 vents, one out the belly skin and one next to the gravity fill. I installed a ball valve in the vent that comes out of the belly skin. When we are traveling with a full load of fresh water to dry camp, I simply close the valve after I see it running out when filling. Now we have a full load of water when we get to our destination.