Get some ice chests; buy or borrow. Fill them half up with water from the aquarium and catch as many fish as you can. You're going to have drain a heck of a lot more than 60% of the water so catching all the fish shouldn't be a problem. If you have more than a couple inches of water in that tank when you try to move it, you might break it.
Save as much of the water as you can, but don't overfill the ice chests. They'll slosh and spill out fish if they're too full. But try and save some water in plastic garbage cans. The more you save, the less you will stress the fish and the more good bacteria you will have. 20 miles is not very far so don't worry about air pumps and heaters. Just get the tank where it needs to be as quickly as you can, dump back in as much of the old water as you can and everything should be OK.
Many good lucks,