I'm no mathematician, so this is an estimate derived from retail stores selling propane tanks that size and you're looking at something that weighs between 1,700-lbs. and 1,850-lbs.
There may be a "tare weight" written on the tank. LP gas weights about 4.24 pound per gallon. Most tanks are not filled past 80% to allow them to breath when the LP expands.
800 gallons x 4.24 = 3,392 pounds plus the weight of the tank