The brass fitting is available at most RV supply outlets. It connects right in line to the water pump. (mine was under the dinette in my Keystone trailer)
The fitting has a valve, and connection port for a hose that you stick right into an antifreeze bottle. You install the fitting once, and it stays there. In the winter you can quickly connect the hose, put it in the antifreeze, open that valve and use the water pump to siphon it into the water system.
Not sure what your dealers kit has, but maybe this fitting is already installed. Check for a water heater bypass on your water heater as well. (couple of valves/lines) Most new RV's come with them already installed these days. (there are exceptions, however) You will use the bypass to avoid filling your water heater with antifreeze.
Pete (Jolly Mon)
2014 Jayco Whitehawk 24 RBS
2004 Nissan Titan LE, 4x4