20 gallons seems excessive. When running the coal over a screen with water before it goes into the truck it was able to drain out. After about 3 4 minutes you would tip it up and probably about 250 pounds or more would run out. At that point you might have another 200 pounds of water still in the truck but its going to vary according the coal.
It comes out of the breaker soaking wet and from there it varies by the breaker and the season. At Hudson for example at this time of the year it either goes onto the stockpile, into a truck if it's off the edge of the pile or into a special hopper for loading delivery trucks where it's washed again.
If the coal is from the center of the pile or off the stock pile where it tends to be more dirty the extra washing at the special hopper that removes a lot of small stuff will easily offset the water weight especially on larger sizes like nut or pea.
You can ask your deliver person about getting dry/drier coal off the stock pile but it will most likely have a lot more fines and dirt in it. If it's completely dry you'll need a hose to wet it down because you're going to have a cloud of dust in your basement thick enough to cut through and that is not an exaggeration.
Having said all that the water weight is accounted for and the cost will be averaged into every ton, if they were selling just perfectly dry coal all the time the cost would go up.