Like many, water is a precious commodity in our van ('17 pop-top 4x4). We custom designed our rig and included two 18 gallon water tanks.
The only issue has been the top inch+ of water in the tank is lost through the overflow hose which is attached at the top side of the tank. If going down hill (hole is on the front of both tanks) this is made worse through a siphon effect.
I addressed this on my sail boat by installing a vented loop in the drain hose. I did the same thing in the van.
I rerouted the drain hose up behind a cabinet (about 1 foot above the tank), installed a vented loop with anti siphon valve and then ran the hoes back out the bottom of the van where the old vent hose ran.
Problem solved. Tank fills to very top and no siphon of any kind. Water overflows out the hose when filling and when filling pressure stops the valve opens and lets air in preventing a siphon and leaving tank completely full. Easy fix using marine vented loop which can be purchased at any marine store.
See:
https://www.boatus.com/expert-advice...n-valve-basics
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