It is under the bed. There is an access door when I move the mattress. That is where I access the water pump, fresh water tank drain, outside shower shutoffs, and winterizing tube/valve. It is definitely in the dark, but not all growth requires light. While the tank is not clear, it is translucent. While we have very good water here, I would not go so far as to say it is totally devoid of nutrients.
We went to Alaska this summer, so we left the MH sitting for 2 months or more without using it in July and August in TN. When we got back, the water coming out of the taps was a milky/cloudy white. I looked at the fresh water tank, and you could see it in the tank. I drained the tank and water heater, and bleached it according to Winnebago's instructions. (I do that every spring anyway) and it cleared it up.