Located in the same area as your pump, so likely in a cabinet, you will have a bypass valve that will draw from that antifreeze inlet on the side of your trailer rather than from your fresh water tank. You will need a section of hose that you can cut just so its long enough to drop into your antifreeze jug. When you activate the pump it will draw from the jug. As mentioned, there are excellent youtube videos to show you how to winterize....lots to mention in a reply on this forum.
2016 Chevy Silverado 2500 Duramax
2016 Rockwood 8289WS, Diamond Pkg.