The way this thread is going, it looks like that 'future' is rapidly approaching!
Since the tanks are from an outside source, try getting a scale for one of the tanks and have a tank delivered to it (or moved to it). Maybe you will have to make room somewhere, or put it in an inconvenient spot. Try it out to see how well it works.
You may even be able to rent a scale for testing.
Your product supplier will likely know how heavy a full tank is, and maybe an empty one too.
If your tanks are currently placed on a stand of some sort, maybe putting some strain gauges on the stand would turn them into a scale.
The strain guages measure the deflection of the stand, which is proportional to the weight.
I have also seen tanks for several thousand gallons of product being weighed by using a balance. They rest on a platform (or stand) mounted on a long bar. The pivot point is close to the tank and the bar extends several feet past the pivot. Then enough standard weights are hung on the far end of the bar to balance the tank weight. Think 'balance scale'.
Cheers,
Tom