Was it a fast leak, or slow leak?
Unless there is a crack in the tank, the only way it should drain relatively quickly (other than through the pump) is the low point drain for the tank itself. There's usually a length of pipe/tube from the low point drain on the tank, usually to a threaded cap which can be removed to drain the tank. I'm thinking that tube/pipe might have come loose at the tank end. Look under the trailer for this drain pipe, then follow it up to the tank (you'll need to remove the underbelly cover if you have one).