Something like this is a good stainless steel interface that can then accommodate a variety of sensors. Assume the tank is plastic? Not sure how thick, but I assume this could be epoxied into the tank, or a plastic block that was drilled and tapped could be epoxied to the tank and then this could be threaded in?
Seems like the main PITA would be the effort to access/remove the tank if needed.
Schneider at 225 4" inch Stainless Steel Temp Probe Sensor Bulb Well 1 4 1 2 NPT | eBay
Then you could slide one of these into the cavity. Fill the cavity partially with RVC and insert the sensor.
This assumes the sensor values are close, if not could be adjusted with a resistor. I assume the temp of the fluid is not ultra critical, probably determines how much is used, but a few degrees here or there is probably good enough?
Sounds like someone should consider a "kit" because it seems these are not uncommon problems.