do you think the drain is plugged or froze? is the valve not opening. there should be an access pannel to the valve. You can back flush with a garden hose if you know which hull port is for that well
Sounds like a mouse nest. If you have a tiedown strap anywhere near the drain hole a mouse may have built a nest and been wintering in style. If you catch yourself a mouse… try using it to catch a CATFISH.