I've looked at the Tank Tender. Very simple device but I think their price is outrageous. the Solo device does not work on metal tanks.
A water meter seems like the simplest thing to install. I know we hold 225 gallons, so if I can monitor the usage I should know what's left without having to worry about tank shape.
Haven't decided yet, still looking. I think I can build a Tank Tender device for less than $100, low pressure gauge, squeeze bulb and some tubing. But then I would have to cut an opening into the tank and all of that. The water meter I just put into the line coming from the tanks and I'm done.
Bob