These gauges work on pressure. Since the pressure in a propane tank is almost constant until the end, these gauges are not really accurate until you are close to empty (as you pointed out). The temperature affects the pressure more than the actual propane level.
There are other methods to determine the tank level; 1) Measuring by weight, which is not practical when the tank is mounted on the TT, or 2) a heat sensitive magnetic strip that attaches to the tank. It lets you know the level of liquid in the tank by heating the strip with hot water. The tank is "cooler" where the liquid is located (or, more accurately, the tank absorbs more heat where the liquid is located).
Check out the discussion on this link: http://www.ehow.com/about_5428024_pr...re-gauges.html
Luckily, we have 2 tanks on our X17Z so it is not too much of a concern.