See level monitor issues

Who installed it? How are the tank sensors ties to the serial loop, wirenuts? Look at the panel, then the sensors. You'll likely find a dangling wire

The Seelevel system only has one wire from the panel that daisy chains all the individual tank sensors together as they are addressable, and a ground at each sensor. You will have one wire that goes from tank to tank and it will be connected to the blue wire at the display.

The seelevel system has diagnostics and troubleshooting steps in the manual, here is a cut and paste....

2.
If an open circuit for all the senders is showing, it is most likely
a wiring open circuit or display failure, since it is unlikely that all
three senders are bad. Try shorting the wiring together at the
display, the display should indicate a short circuit. If it doesn’t,
the display is bad. If it does, then remove that connection and
short the wires together at the sender locations. If no short
circuit is shown, then the wiring is open. If the display does
show a short circuit, then the senders must be bad.

We were parked and everything was fine. We have a problem with one slide out. It is uneven and we were trying to figure out what was happening. When we were trying to retract the slide, the smoke detector chirped and we noticed that the see level read outs no longer worked.

Wonder if you ran over something and cut or pulled the wires to the tanks. I just replaced mine. My case. Animal's chewed them off.
Crawl under and look. Should be 4 wires to each tank