[RMcNeal]

I would drain it any time it will be 2 weeks or more of non use. In the summer months, bacteria can grow in the system.
We never drink the water in our tank or system. I carry bottled water for drinking and cooking, but you still get some when showering and brushing your teeth. I usually only carry about 12-15 gallons in the fresh tank for flushing and hand washing on travel days. We normally have full hookups and only use the fresh tank during travel. Water in the water heater will get stagnant pretty quickly, so I will typically drain it when we get home if I don't plan to use the rig for the next couple of weeks.
I will add that I also sanitize the system twice a year in the spring and fall.