I recently installed Safiery Tank Level Sensors IP65 on grey and fresh water tanks using the Cerbo Gx Tank 140 for voltage. The install and calibration went well. However, when I went to use my RV a few weeks later (all tanks were empty), all three sensors would not read voltage. Each sensor was displayed on the Cerbo device list, and the status displayed "OK." I reset each sensor, and re-calibrated each (the new empty voltage now displayed was different than the first setup). Each sensor seemed to be working properly. After the trip, and all tanks were emptied and left that way for several weeks, the same thing occurred. The sensors status displayed "OK," but they did not read voltage (voltage did not change as water was added). Any ideas on what is going on?