Holding Tank Wiring - Test wires

These wires are definitely connected directly to the sensors or sensor wiring on the tanks, not running lights, although the colors might make one think of running lights. When I had my tanks exposed a couple of years ago, I noted that the wires were attached to the tank sensors but did not document exactly what was attached where. I have seen some material saying that different levels in the tanks will produce different resistance in the sending wires. The question is how to use these wires to do that testing? I know this forum has some pretty adept people when it comes to electricity and likely they have the correct answer.

I'm also wondering if trailer manufacturers other than Northwood/Arctic Fox provides these wires so that troubleshooting might be made easier (if only one knew what to do with them)??