Like Lou said, Clorox bleach is your friend.
I'd flush it once with clean water and then do a couple sanitize cycles.
What you're finding is probably either mildew or algae.
Both can be taken care of with chlorine bleach.
Add a cup of bleach and fill the tank to the top.
Let it sit for an hour and then drain and do it again.
The 2nd time once it's full run your pump and open all faucets
one at a time. Remove any faucet air-raters if you can as they may
clog with debris.
Once you've got it cleaned fill and flush with plain city water.
That should remove the chlorine smell. A slight chlorine smell won't
IF you are leery of drinking from your tap get a blue water tote jug
called Aqua-tainer by Reliance. These can usually be found in the
RV/camping isle of major dept stores.
They come in 2 sizes.
We have both and use them on occasion for our coffee and drinking
water but sometimes we just drink from the tap in our trailer.
Don't forget to flush the hot water heater if you have one.
PS- A cut in half washing machine water hose makes 2 great water tank
fill hoses. Often a garden hose won't fit in the fill opening but a hose
with just a plain cut end will fit. I replaced my washing machine hoses
with stainless steel braided hoses and saved the old black rubber hoses.
They come in handy for filling buckets from a faucet or attached to
my garden hose I can easily fill my tank.
You can also remove the drain plug from your water heater and stick
one of these cut hoses in there to flush it out.