360DA Fresh water tank - additional line running back to tank

[scottrreiner]

I have a 2005 36 Sundancer and am having fresh water pump problem (this post is not about the pump). While checking connections, I noticed that I have a line that splits off the output of the water pump and goes back to my tank. There is a valve that sits on top of the tank. None of this connects to the fill line. Can anyone explain what this line is for and why the shutoff valve there?

 

[my3sons]

I have no idea, but can tell you that my 2003 did not have that. Previous owner installed for some reason??? Something to do with a DIY winterizing method??
Interested to hear what others say