The sensor system
The panel has a power and ground connection.. The bottom sensor on the tank is also grounded, MAY or may not be grounded via the panel ground (No reason to do it that way but who knows)
I am showinga "3 sensor" type, some are 1/4, 1/2, 3/4 F
/\/\/ is a resistor
The resistors are inside an epoxy "Chip" which is likely wrapped in tape Wires from each sensor go to this chip and then one goes off to the panel
Testing.. 2 person job
Have partner inside talk to you via cell or 2-way radio
Take a jumper wire (Any length of wire will do, helps if it has a clip on at least one end) and hook it first to the bottom (Ground) sensor, this is the one sensor NOT hooked to the aformentioned chip
Other end of wire to known ground (Chassis) and have partner check.. WORKS, bad ground, sensor fix the ground
IF still no work then touch the FULL sensor.. WORKS, bad sensors (Nearly zero percentage posibility but the test only took a second)
Now, find the wire going to the panel, expose the wire and touch it, FULL TANK, bad chip (More common but still rare)
Now seal the exposed part of the wire (GOOP works well) and go inside.. Take your jumper with you
Open panel and find the sensor wire.. Test at that point, WORKS, if shorting the wire to ground (use panel ground) works you have a broken wire or bad connection
Suspects include the crimp on splices where the panel's tiny wires hook to much larger wire used in the motor home.. I had to re-do several of those in my house.
Home is where I park it!