To get antifreeze into the pump, if you don't already have one you'll need a
pump winterization kit like this one (Amazon). It's also very helpful to bypass the water heater when winterizing (uses A LOT less antifreeze). If you don't already have one on the trailer a
water heater winterization kit like this one (Amazon) will do the trick.
The tube coming out of the freshwater tank is used to drain the tank as much as possible. There are two other tubes underneath somewhere - one blue, one red right next to each other. Those are the low point drains for draining water/antifreeze out of the plumbing.
The small amount of water in the bottom of the freshwater tank (you'll never get it all) won't cause problems - it has plenty of room to expand without bursting anything. The same for the small amount of water in the bottom of the hot water heater.
Don't forget to fill the traps (sink drains) with antifreeze too!