They may not be connected
Whether they are or not, most water tanks don't have a drain...they can only be drained through the faucets, which leaves about an inch of water in the tank. If the tank has an inspection port that provides access to the inside of the tank, that can be removed with a shop vac or hand pump. Otherwise, your only safe option is antifreeze.Use only non-toxic potable marine/RV antifreeze and follow the directions in the article "Winterizing Plumbing" in the Head Mistress forum reference library (link is on the forum homepage) to winterize both your fresh water and sanitation systems.