All tanks I have ever had filled were done by the gallon.
I have a 1000 gallon tank at my home. Not very practical to weigh a 1000 gallon tank.
Propane delivery trucks all have meters that measure the amount of propane pumped into consumer tanks. These meters include a volume correction device known as an automatic temperature compensator. The temperature compensator takes into account the temperature of the liquid propane running through the meter and automatically adjusts to correctly deliver the amount of propane that the consumer ordered. By law, these devices are required to be re-calibrated and are adjusted based on the temperature of the liquid at the time of calibration. When a delivery of propane is made to your home or business, know that the amount you paid for is the amount you are actually getting.