If any one sensor fails open the full LED won't light when the tank is full. If any two sensors fail open the only LED that will light when the tank is full is the 1/3 one.
The level that causes the 1/3 LED to light is a function of which two sensors fail.
The 1/3 LED will light when the remaining good sensor sees liquid.
For example:
If the full sensor is good the 1/3 LED will light when the tank is full.
If the 2/3 sensor is good the 1/3 LED will light when the tank is 2/3 full.
If the 1/3 sensor is good the 1/3 LED will light when the tank is 1/3 full.
If any sensor fails closed then the 1/3 LED will always be lit. If any two sensors fail closed then the 1/3 and 2/3 LEDs will always be lit. This isn't what you are seeing.
The display board uses an LM339 Low Power Low Offset Voltage Quad Comparator IC to sense the voltage change when a sensor closes or opens and turns the proper LEDs on the display board on or off .
It would seem that you could have a problem with either a couple of bad sensors or a bad display board.
You could narrow it down by checking the voltage on the wire going from the sensor string back to the display board and seeing how the voltage changes as the liquid level go up. The voltage should drop some as each sensor closes. If you see three distinct drops as the level goes past each sensor then I would think the board is bad.
Before I did anything though I would unplug and re-plug the connectors on the board and at the tank to make sure the connections are good.
Lichtsinn Motors, a Winnebago dealer a couple of miles from the factory is a good place to buy the sensors or board.