If these are cheap sensors, how about sticking one 10 mm above the max line, one on the minimum level line and use the circuitry to compare the temps. Assuming the sensor next to the liquid is hotter than the one next to the steam at the top, when the min one's temp falls to around that of the sensor above the max, time for a visual check of the levels...
The sensor is a small square block type about 40mm square. They do not monitor temperature, they are for level sensing. I am fitting it instead of using a float switch in the tank, same price as changing the tank to a new one with a float switch and considerably cheaper than buying the kit.
It sits below the seam on the tank, butting up to it nicely.