On our 06 Winnie Outlook 29b, there are 4 low point drains, in the water pump compartment there is a big yellow handle for draining the fresh water tank and on the pressure side of the pump there is also a low point drain in the plumbing supply line to the coach. There are also 2 low point drains, hot and cold. They are located in an outside storage compartment and go through the floor of the basement compartment. These three are the same type of valve 1/4 turn white handle, the same as the winterization suction valve.
It's possible that when changing the valves in the area of the fresh water tank, you've may have forgotten to close this one.
Ken