I second the ultrasound rangefinder. Looks like you already have an opening in the top of the tank, so no new penetrations would be required. It's a seasoned technology, nothing new to invent.
Another option not yet mentioned is the fact that liquid in a tank generally results in a temperature difference in the wall of the tank, with a dividing line above and below the liquid's level. Some installations I've seen have instrumented this, and you could also literally view it with a handheld IR imager. The latter approach would require no installation and zero time to implement once the imager was purchased. Also no moving parts and zero maintenance. Resolution and accuracy would have some human factors but on a tank that large a few percent may be sufficient.