If you look at the pump, you'll see that 1/2" hose is attached to a fitting and the fitting is inserted into the pump head and locked in place by a blue "slide lock". To introduce antifreeze into the plumbing lines, you'll need a fitting and a length of clear plastic tubing. I use 4' of tubing. You can order the fittings on Amazon at this link:
https://www.amazon.com/Flojet-Port-D...9263696&sr=8-5
Pull that blue "slide lock" and wiggle the fitting out of the pump head. Insert this (the same fitting) with the hose attached, put the "open end of the hose" into a gallon jug of antifreeze and turn the pump on. As you open faucets, the pump will draw antifreeze from the jug. Once the jug is close to empty, place the tube into another jug of antifreeze and continue until the trailer is winterized. Then, remove your "winterizing set" and reinstall the original 1/2" hose into the pump head and lock the blue "slide lock" in place to secure the fitting.
You may also want to consider protecting your pump from "trash" that's almost always left in your fresh water tank by the factory. Those shards of plastic and sawdust will destroy your pump head in short order if you're not careful. That almost always seems to happen on a Saturday afternoon, miles from the nearest place to buy a replacement pump and that store is closed until Monday.... Of course, it only happens on the weekend when you've really got something special going on.... Buy the strailer to fit between your pump and the fresh water supply line...
https://www.amazon.com/Flojet-Port-D...9263696&sr=8-5