I would suspect that it is a Workhorse chassis that has the intake horn right in front of the radiator, left side, when looking under the hood. It can get water in it from any water that got in from the front when the coach was being washed or from rain. Obviously if you had water in the air box, the coach was exposed to water at some point when it got in to the air intake opening. I doubt if it came from the windshield washers because the volume just is not there.
Some coaches, because their front end was not designed according to the chassis manufacturer, do suffer from rain intake during a heavy storm.