Originally Posted by TheWolfPaq82
I must chime in on the fact he stuck a bolt in the vent to prevent "Siphoning" of the tank. By design...it would be physically impossible for the FW tank to siphon itself as it would require that the overflow vent extend all the way to bottom of the tank inside for siphoning to be possible. Tanks are not constructed this way.
In reality, the vent tube can become a Venturi tube when driving down the highway and can start a siphon based on the angle of the tank when connected. Many campers do not ride perfectly level and based on how the tank sits at towing attitude can prevent the tank from filling completely and in transit can actually suck out a few gallons of usable water.
In the photos you can see that the tank only indicates 1/2 full on the lights, yet the tank will not accept more water due to the angle and location of the white fill hose and vent clear vinyl hose. You would think that disconnecting and tilting the trailer back towards the rear while filling would give you a full tank, but when you reconnect the water will just come back out. It is best just to put as much in as you can while connected and fill some 5 gallon water bottles to top off the fresh tank when you get level. They will come in handy when it is time to refill the tank without moving the camper.
I made a fill hose for 5 gallon water bottles from a piece of vinyl tubing and a water bottle cap. (photo) A tiny air hole will prevent the bottle from collapsing as you fill the gravity water fill port.
Plugging the vent line will prevent the Venturi effect when traveling, but will also prevent the water from flowing if left installed when you run the pump.