use of fresh water tank

I have 2008 Class C (31') Sunseeker which I have never used the Fresh water system. When I tried to fill the tank the water ran out of a plastic tube located in the right rear of the the rig. This tube is open to the atmosphere with no cap . As the water filled the tank the water ran out this tube. Was or is this tube suppose to have a closed end to it .

There is no instruction specific to my Rig that explain anything regarding the fresh water system.

Please help

Frank