Drinking water from a Westy tank should be no problem as long as the tank is
clean and the water is from a reliable source. The tank itself is often
cleaner than the storage and delivery system from the source to your
house...and we have the added benefit of visual inspection of these tanks
upon removing that big lid.  :)

If the tank does "go bad", you can immediately detect same by taste and/or
odor. Cleaning is easy and once a year is good practice:

Drain the tank completely then refill with fresh. Drop in 50 ml or just less
than 1/4 cup of household bleach mixed with one gallon of water. Top off the
tank. Drive around the block or "shake your van" (okay, no jokes here). Turn
the faucet on & run one quart of water through (to sanitize the pump &
line). Let stand several hours. Drain completely.

To remove the bleach odor, mix 1/3 cup of baking soda with one gallon of
water, drop into tank and fill tank with fresh water. Drive or shake, run
the faucet again, let stand for several hours. Drain tank. That's it.

As was suggested, it is good practice to empty your tank between uses unless
you are using it frequently. This prevents possible bacterial growth and
reportedly extends pump life.

The source of your water is of course a determining factor for quality. If
you are ever in doubt, don't put it in your tank and drink bottled water.
This is critical for those who are immune deficient when it comes to things
like cryptosporidium parvum or giardia lambria.

Kit Carter, 85 Westy
aging water treatment operator in CA