Tank levels, always a guess. I don't know why they can't get a good system out there. Maybe the new trailers do, mine did not.
I installed a Vena tank monitoring system, a Bear to calibrate, and I think this led to the demise of that brand, but there are others out there.
I love mine, the LCD screen is attractive and easy to read and use, it also has a very accurate voltage meter on it.
I will post a link to a similar unit:
http://www.ahoycaptain.com/shop/scad_tm2.html
Here is my Vena: