First are you sure the drains are shut. If so you most likely have a broken pipe.
I would make up an adapter, or buy one, for the water fill point and supply air at a regulated pressure to the system. 30 lbs or so, and start looking for the leak. You will hear it. You just have to get in there and look.
From your description of just draining the system the first place I would look is the head of the pump because draining will not remove the water from it. Second is did you drain the hot water heater?
In general draining alone will not winterize your trailer. You need to either blow it out with air, one fixture at a time, or fill it with antifreeze. I do both because the plumbing is not installed with drains at all the low points. Also there is no other way to get the water out of the fill point than air. That section of pipe is remote to the rest and you have no way to brake the vacuum on that section of the piping. If you just drain and fill with antifreeze that section will still have water in it.
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