Each gallon of propane weighs around 4.2 lbs and in a full 20 lb propane tank you'd have 4.7 gallons of propane.
So in a 100 gallon propane tank you'd have 96 gallons of propane due to expansion because a propane tank is never completely full.
So since one gallon of propane weighs 4.2 lbs the 100 gallon propane tank should weigh around 400 lbs in weight once filled.
The propane tank will only have around 96 gallons of propane added to it when full because of safety and expansion.