how accurate are the sensors and propane sensor for levels

If you're using bottles, get one of these. Uses sound to detect the difference in density of the liquid and gas. Any of those stick-on detectors require a temp difference and you only get that when the gas is flowing.

If you have a large built-in tank, the level gauge is probably a float type and should be accurate to within a 1/4 tank. Remember that a full propane tank reads 3/4.