Hi Rob,
Low point drains can cause problems in cold weather. The drain valve is exposed to outside temps and there's an inch or two of standing water above the valve that is also exposed. When temps drop below freezing, the water above the valve freezes and the ice can wick up into the tee, blocking the water line.
Your fresh tank drain has that exposure and many people camping in cold climates either heat the exposed line, or insulate (or both), or push the drain hose up above the coroplast.