Fresh water tank under camper-Camping in colder weather.

Seriously, you never drain the fresh water tank. I assure you there is a lever or valve somewhere to drain it. It would be dangerous to leave water in the tank indefinitely I would think; bacteria, etc. eventually. I drain mine after any use unless I'm going back out within a week or so. Re check the manual. Am I missing something?
If a water tank is full, it would take a long long time of really cold weather to freeze it but not the case with water in the lines.