I can put 500 gallons of water while it sit at home. I don't think it can carry 500 gallons of water and tow since M1102 rates as 4,200lbs. 1 gallon of water weight 8lbs so 500 x 8 = 4,000lbs. So the total will be 4,000 + 1,400 (trailer) + 500 (tank) = 5,900. It is 5,900 - 4,200 = 1,700lbs...