Go to a pool or spa store. They have a product named Di-Clor. It is granular. Take note of the water level in the fresh water tank, then empty the tank. Take about two table spoons of Di-Clor and put them in the tank. Fill tank to a level higher than before you emptied it. Run water through every potable water source (kitchen sink, shower, bathroom lavatory, even the faucet in the black water underbay - if you have one) until you can smell the chlorine released by the Di-Clor at each of those points. Then empty the tank again before refilling with fresh water.
Just some supplemental info to Will's above comment:
Plain old household bleach is about 6% chlorine, less if you leave it in the sun. (Sunlight degrades the chlorine very quickly.)
If you pick up liquid pool chlorine in the grocery store, Home Depot, or a pool/spa store, it's about 10 - 12% chlorine.
Di-Chlor in granular form is 55% - 62% chlorine, depending on the manufacturer.
The REALLY strong stuff is Tri-Chlor. It runs about 99% chlorine in granular form. It's what we in the industry use to turn a "swamp" back into a crystal blue pool again. Very slow dissolving, which is why it's used in floater tablets. Way, way too potent to use in this application!!
If you decide you want to use the Di-Chlor method, I would suggest you dissolve a couple tablespoons in a bucket of water first, then pour it into the fresh water fill. It dissolves quite rapidly, especially in warm water.
Be careful about spills - you'll "bleach" your clothes!!
You can buy Di-Chlor in 2 lb bottles for spas, or in various-sized buckets for pools. Chlor-Brite is Leslie's house brand, they also carry 1 lb bags for "shock treatment." (Advertising hype, really. . .) Other pool stores should have the 1 lb bags, it's a common industry staple. Even tho it's more expensive per lb, I'd recommend buying just the 2 lb bottle for spas, small enough to leave down in the bay out of the way. Just make sure the cap's on tightly, or the fumes will corrode everything around it! Better yet, seal it up in a gallon-sized freezer zip lock bag.
FWIW & HTH. . .