Resistive type fuel level sensors which are used in fuel storage tanks(such as those at fuel stations) comprise of 2 visible parts; dual probe rods and a connecting float.
When the float is at the very bottom the output of the sensor is Vmin(possibly ~0V) and as the float progressively approaches the top the output increases upto Vmax(possibly ~Vsupp).
I would like to understand how these sensors work possibly by a simple circuit model?
I'm taking a guess based on the type your describing. The long probes with a magnetic float has a resistor tree wired in series with reed switches.
It has been a while since I pulled one down for repair but believe each resistor / reed was wired in parrellel. When the magnet was over a reed. That circuit was closed and the resistance of the sender decreased. Usually in the range of 10-220 ohms for the ones i dealt with.
Here is a link that has pictures to go with my explanation
A 500mm long sensor had 22 switches from memory. Its not linear but steps.
Some car manufacturers such as ford took this a bit furthur and included an amplifier on top of the reed switch sender to convert it to a voltage for the engine ecu.