If you think through the physics, you'll agree that weighing is the most accurate way to determine what's left; albeit not the most convenient method.
Propane is a liquid that vaporizes into a gas when exposed to a sufficiently low pressure, such as atmospheric pressure. At a given temperature, the pressure produced by the liquid propane is a fixed value - regardless of the amount of liquid in the tank. This is called its 'vapor pressure'.
When the tank valve is closed, liquid propane evaporates until the pressure inside the tank equals the vapor pressure value (for a given temperature) at which point it stops evaporating When the valve is open and you're using fuel, the pressure inside the tank drops and liquid starts to evaporate.
For any liquid to evaporate, it must absorb heat (just like when you're boiling water). Propane absorbs heat from the environment making it colder than its surroundings (which is why the liquid part of the tank feels cool).
This is what leads to the weaknesses in the "gauges" available on the market today.
The 'pressure' gauge only changes meaningfully when the liquid is virtually gone.
The 'temperature' gauge only changes when the gas is being used, and at a rate fast enough to noticeably change the liquid temperature.
There's some good advice here, and it's what I use:
Have two tanks with the automatic switch showing when one tank has gone empty. If you can go at least one day on a single tank, then a daily check is sufficient to see if one's empty and you're now on 'reserve'.
Keith & Liz
2010 SilverBack 33L