I think I would just fill it with tap water and then treat the water as it was used, employing some of the standard wilderness/backpacking methods like Aquamira, a Steripen, etc. or just plain old boiling. I would be inclined to change the water on an annual basis and check carefully for any algal growths.

That is a massive tank! Frankly, I would prefer several smaller containers that would be more portable.

You might see if you could submit samples for testing to see if any bad stuff appears. My experience is that often stored water can remain drinkable for at least a year, but you would want to be sure.