In my Trade Wind, water tank is under the front window with the fill cap on the front of the trailer on the street side. The tank is located inside above the floor and covered with thin plywood. On either side of the tank are two small compartments fitted into the curve of the front corners. The water pump and a tangle of plumbing and valves reside (resided, no pump for some time) in the streetside compartment.
If I remember correctly, and it has been some time since I've used the Trade Wind, one of these valves allows filling the tank. The others are drain valves. Check the valves there. Hopefully you will find the open fill valve there.