I typically go overboard when winterizing by blowing out all the lines with air and then running the bypass water pump converter which draws the antifreeze from the bottles thru the entire water system. I just turn on/off everything in the trailer including outside shower and sink.
To add to the process, I recently purchased a hand pump to do the external black tank inlet plus main water inlet. Attaching a photo of pump. Think Amazon has them real cheap now.
I like the idea of using the external shower hose to do same thing but took the easy route and just bought another tool.