Two more considerations that I did not see in the previous replies...

1 - Water is heavy. A 300 gallon tank, full, will weigh in the
neighborhood of 2500 pounds, concentrated in just a few square feet.
This can easily collapse a roof if it isn't supported properly. Those
storage tanks you see in other countries are put there when the house
is built, usually, and so would have proper support underneath. Houses
are often made of concrete too in many countries, e.g., in 3rd world
contries where storing water is common. And fall down quite a bit more
often than in the US.

2 - Stagnant water will not be drinkable after a few months. At least,
you won't want to drink it, because it will be full of algae, crud,
slime, etc. Periodic flushing and refilling aren't really great ideas,
because eventually it will fall out of habit, or be neglected, etc. The
best idea is to just use the water continuously. E.g., find some way to
make your system such that in regular use, all water gets flushed
through the system daily. The reservoir, in other words, should be your
normal, daily water supply, replenished as needed from city water. When
power goes out, you just cut off the city supply in some way, and rely
on the water remaining in the system at the time the power was cut.

-Kevin