Hi, I'm not sure if you would want a constant level status (e.g. 0 - 100%) or as stated simply a low and high.
I've worked on some grain silos, and some folks used a product called a "Bindicator" which was merely a diaphragm located on the side of the silo. One cut a 4" hole in the silo and mounted the device. This was on corrugated metal silos, not concrete or anything very thick. The pressure from the height of the product pushed on the diaphragm and activated a switch. Also on top there was a bindicator that used a fractional horse motor with a torque switch. The motor had a gear reduction which turned a variable length rod that had paddles attached to the bottom of the rod. The rod hung down into the silo and turned slowly (fraction of an RPM) and if the product got to the paddles and caused resistance for the motor to turn, the torque switch made and set off an alarm, or an input to stop the filling process, etc. It was not a very large device, but you would probably need AC up there. So the diaphragm switch for low level and the other for high level. Hope this helps.
Ooops!! TWS beat me to it, I guess.