The attached photo shows the level sensors on the grey tank of my x17z. (The black tank has the same set-up.) When the fluid in the tank reaches the probe extending into it at a certain level, an electrical signal is sent to the indicator (receiver) you view inside your TT.
Recently I stopped getting a reading on my grey tank, although the lights in the receiver were still working fine - they would flash when pressed. The probe portion of the sensors rarely fail, so I checked the wiring. The problem was a bad ground wire connection which is easily repaired, usually by cleaning or replacing, and with little or no cost involved.
Anyway, while I was under the TT, I took a picture of the sensors since they seem to come up in the forum every so often as an item for discussion.