I hated seeing this. The RKL was perfect for my application. Temp control, timer for protein skimmer, and pump pause for feeding. Have three tanks running this way, and now have a temp probe failed. Cannot afford to replace with new system. Sucks for sure.
Did you check on ebay? I think I seen a few for sale there. Adafruit has a sensor that may work It just don't have a long cord or the right connector.