Here’s a remote. Your tank likely has a mechanical float gauge that turns a small magnet at the top. The magnet in turn operates a gauge capsule that is set on top of the float gauge. That gauge capsule displays the tank level, but remote reading capsules have a potentiometer winding in them that allow for remote electrical gauges to be connected. This is nearly identical to how many marine fuel tank senders work with electrical sender capsules.
Using a Z-Wave sensor like the Fibaro universal sensor or perhaps the Fortrezz multi IO sensor, I suspect you could get a tank level reading from such a remote reading capsule.
You might also be able to place a door/window sensor near enough to the tank’s float gauge that the magnet might trip the sensor at the right level.
I have never attempted either of the previously mentioned solutions, but I’m pretty sure I could get at least the first one to work. The second idea would depend on how much space you have to work with around the float gauge and if you could mount the door sensor at just the right spot and angle. You’d need lots of duct tape, I suspect.
Edit: From your link I found these gauge capsules. Now all you need is a Z-Wave sensor that can take the variable input, like the Fibaro .