1/4 cup of bleach per 15 gallons of tank capacity is the correct ratio.
Ace owner's manual wrote:
Prepare a solution of 1/4 cup household liquid chlorine bleach (5% sodium hypochlorite) to one gallon of water for every 15 gallons of tank capacity.
Although my coach does not have a gravity fill, I use the hose method through the city water connection (valve setting for tank fill.) If your gravity fill is high up, the bleach might run out of the hose as you connect it to the source faucet. You can handle this by attaching a small shutoff to that end of the hose.
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Tom