As stated above, to completely drain the fresh water system, you need to open the Low Point Drains. These will be 2 Pex lines, possibly Blue and Red, hanging down from your trailer. They should be capped from the factory.
First, open all the faucets (And place something heavy on the foot pedal to your toilet...) in your trailer, and, the pressure relief valve on your hot water tank. (Don't forget the outside shower, if you have one...) Then go outside and remove the two caps from the Pex lines (See attached photo). After all the water drains out, you can, if you have one, screw a "blow out" plug in your city water connection, and blow any remaining air out with compressed air. (Use a reduced pressure on your air compressor...I recommend not more than 50 PSI, with everything still open!)
Hope this helps!