Please excuse my ignorance of all the terms.
It is gravity fill where I stick a hose in the hole.
Where would the vent be, that you are mentioning may be blocked?
Please correct my assumptions below:
1. Fresh water gets filled from the top of the tank.
2. There is port at the bottom of the tank that connects to the pump. The pump then connects to the cold water line inside.
3. I would assume that there is another port on the bottom of the tank that is for the drain? I'm not sure how else the tank could get emptied when being winterized.
4. If #3 does have a port, where would the valve be to shut it off?