WELCOME to the group! There's lots of good friends to be found here, and tons of experience.
Even if you get the system working, the next time you dump, a piece of TP will stick to the sensor and show you FULL, even though you know your not. It's the nature of the beast.
There are five screws through the wall into the tank attached to wires on the outside. The bottom one is the ground. As the water level rises, you will reach the first sensor screw. This completes the first level circuit by using the fluids as a "switch" between the two screws, thereby turning on a light on your gauge. As levels rise, each additional screw makes contact to complete the next circuit, turning on each additional light. In theory, a great bullet proof system, right? NOT! All it takes is corrosion, broken wires, or "cling ons" to shut you down.
As already mentioned, it is a lot of work to get to your sensors, to correct a problem that will probably re-occur. Maybe a flashlight pointed down the hole is an easier fix. Just don't drop it! LOL
Steve & Jan, Ava & Emma (Mini Schnauzers):
2016 F350 Lariat 4X4 Powerstroke CC/SB "PULLTOY V"
2013 Alpine 3535RE "MAGIC CARPET IV"
Proud Navy Vet!