How to know if fresh water tank is full?

Lantley wrote:
If I don't use the gauge and fill till it over flows than a siphon will start and my tank will drain itself onto the ground.

I have that problem too, and it makes me insane! I fill the tank until it starts to come out of the overflow, but then if the rig is tail down at all while setting up, it will siphon the tank dry and I can't stop it. Very frustrating when I'm boondocking, as I usually have to set up the RV nowhere near a spigot.

The only thing I can think to do is to plumb a shut-off into the overflow so that I can stop the tank from self-draining until I get level.