As has been said, a lot of water can leak out of the overflow tubes. Though they come out of the top of the water tank, often a siphon effect can happen. Putting a valve on the tube end underneath the trailer will work to stop this. Fill the tank till water comes out of the overflow and once the flow stops, closed the valve. Since your water tank fill spout usually always has a breather hole/tube (Usually, just above and left of the fill spout, just a little smaller than a dime sized hole with a screen), you needn't worry about opening the valve underneath until you go to fill the tank again. The breather tube will allow the tank to breath.
2015 White Hawk 25BHS Glacier Package
2004 Dodge Ram 2500, 5.7 Hemi, 5Spd Manual
2008 Arctic Cat M1000 SnoPro
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