Before you dig in to far lets try and define the source of the water. Locate the water pump and disconnect the output line from the pump and plug it so you can pressurize the system. I would adapt an air fitting to the city water supply and pressurize the system to about 30 pounds. If the pressure holds your plumbing system has not been the victim of freezing. If that checks out OK fill the fresh water tank and check for water in the hole you mentioned. If both the tank and plumbing hold water you may be looking at a sewer leak. To check the sewer. If you have both gray and black water tanks fill them separately and check for water. If everything passes the test you may be dealing with a body leak.
As for the water drain there should be 1 or 2 for the water system and 1 for the fresh water tank. If no one come back with an answer for your year the easiest way is crawl under the trailer and look.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles