What is the metal tube with a 12v positive and ground wire running to it on my leveling jack? Looks like a thermistor or mercury switch possibly?
That tells the jack that it is vertical. Without it the jack would extend horizontally.
That was my guess, but how?
It might not be the vertical sensor after all. The adjustment is in the link below.
https://www.hwhcorp.com/ml29931.pdf
That should tell you the jack is extended.
As soon as the jack swings down you should get a light come on. When the jack is retracted and swings back up the light goes out. It is a mercury switch and the mercury inside makes the electrical connection when the jack is down.