There is no way you will ever get the fresh water tank completely empty. The drain line is not on the bottom of the tank. It's up about 2-3 in. on the wall.
Don't worry about it. There is enough room for expansion if it freezes to not cause any problems or damage.
Am I understanding you correctly that you are pouring anti freeze into the fresh water tank?
Your trailer should have valves near the water heater that you switch/turn for winterizing. When turned in the correct position, the anti freeze only runs into the water lines
. Nothing goes in the water heater or fresh water tank.