winterizing finality

I have winterized the lines of my trailer, the toilet, inside shower, outside spray port, left anti freeze in the traps, toilet bowl, but I just read a blurb yesterday about putting anti freeze in the city water inlet valve. when I depress the button behind its filter screen, some water came out, but I can't figure out how to get a/f into it. Can I suck anti freeze into the inlet side of water pump and backflush line to city water inlet that way, or is there a check valve involved. Any advice would be appreciated as I'm running out of time with this very recently purchased trailer.