KymLou,
Welcome to the forum. I trust you'll be like the rest of us - eager to learn & teach our friends how to enjoy life in a van of some type.
This forum has a wealth of knowledge stored in the various tabs - "Advanced Search" is very handy.
I suggest after you have operated the tap & emptied both tanks successfully - draw a circle on a sheet of paper & mark the 12, 3, 6 & 9 o'clock points with what Jeff has shown above & keep it in your paperwork, manuals that you have for the van.
The tap might be near one of the tanks or 1/2 way between. Generally each tank has a screw on cap on one end that enables you to empty it.
And for any rough road travel that you leave it in the "Supply shut off" position to prevent stones that may damage the pipes/hoses from draining both tanks.
Also check your weights - I have two 95L tanks - that's 190kg of water out of my maximum of 375 allowable kg. You're lucky - you have only 164kg of water on board.
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Warren
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If you don't get it done today, there's always tomorrow!
How about this. 1. Fill Tank at input inlet. This water is the water that comes out of the taps. It flows down the drain and fills the other tank. This is a grey water tank. You then empty that tank either at a dump point or onto the ground. The gauge inside van indicates the amount of water in the water tank. Although, mine show two tank levels.
I have a tank 1 and tank 2 . My gauge is hooked to tank 1 only .So i always use tank 2 first and when the water run's out i will switch it over to tank 1
and then i will know i have 100L left and this is the time my gauge will work . Hope this helps.