Fresh water tank water level incorrect

That's a new one on me. My freshwater gauge works fine but my black and grey gauges seldom do.

Do you still have the Newmar? If so, they use a foil pad type system glued to the outside of the tanks. make sure those copper pads are still mounted to the side of the tank. AT the monitor panel is an adjustment for full and empty. 2006, they may have upgraded already to a different system that programs slightly different. BUT, if you have the panel that has the adjustment screw above each tank readout, you fill completely and turn the adjustment screw until the tank LED's just read full. Doug

Ok I will draw you a wiring diagram each "(S-XX) is nothing more than a screw or nail like rod that protrudes into the tank at the proper level, and /\/\/\/ is a resistor.

Panel-----(long wire)------(S-Full)--/\/\/\/----(S-3/4)---/\/\/\--(S-1/2)

It goes on through another resistor to the 1/4 sensor but I ran out of line.

These resistors are in a small epoxy box, often orange, and often wrapped in tape, stuck somewhere near the sensors (Behind a metal bar on mine).

Now. if the tank shows FULL when it is, in fact, FULL then goes to EMPTY this resistor array has failed. Should cost under 10 bucks.

IF the tank shows FULL and 3/4 but never 1/2 Same thing

IF it never shows FULL, then the line from the panel to the top sensor (Full) or to the array would be my suspect

Sometimes the wire from the panel runs to the FULL pin, sometimes to the array and then to the FULL pin but the FULL pin is always Direct connect to the panel so it makes a great diagnostic device. Note if your sensors only show 3rds,, You may edit 3/4 to 2/3 and so on leave off the "Additional" line.

There is also a "Ground" Sensor pin near the bottom of the tank, not connected to the array it has it's own line back to the panel.

Another test, Using a jumper connect this Ground" sensor to one of the higher sensors, Panel should report that level of water. (OR tank level whichever is higher)

You have a loose wire someplace. I would check behind the panel first, after that your going to have to dig into it deeper to expose the FW tank.

Ensure wiring to FW tank sensors is still intact, possibly emptying and sanitizing would help in case there is some growth within.