Onboard tanks are usually black in colour and are also usually housed inside dark cupboards and as algae needs light to grow there is minimal likelihood of it growing in an on-board tank. Of course there is the possibility of bacteria entering the tank but again the likelihood of it thriving is small due to the lack of nutrients and the usual temperature of the water. Ours comes out of the tap cold, even in the middle of summer. The use of a steriliser like Milton once or twice a year is sufficient to kill any residual bacteria.
Of course if you only use the water for tea, you will be boiling it anyway which will kill bacteria. For cold drinks I always feel it is best to drink bottled water (or preferably beer or wine) as you generally don't know where campsite water has come from
