Some of this comes right out of the Airstream owners manual. It is something I posted some time ago.
If you're using bleach that comes in a gallon jug, multiply the number of gallons of tank capacity by .13 to find how many ounces of bleach you need.
If you're using the new "Ultra" bleach that comes in a 3 QT jug that's supposed to equal a gallon, multiply the number of gallons of tank capacity by .1 to find how many ounces of bleach you need.
For a 40 gal tank, that's 5.2 oz of regular bleach and 4 ounces of "Ultra" bleach. Put that in a gallon container and fill with water to dilute it. Then using a funnel, pour it into the tank filler. Then fill the tank.
Assuming the hot water heater is still empty from winterizing, unbypass it, turn on the pump and open a hot water faucet until bleach smelling water comes out. Do that with other hot water faucets, including the shower. Then go back and do it with all the cold water faucets, including the shower, the toilet, the spray nozzles on the toilet and sink, and the outside faucet if applicable. This gets bleach throughout the pipes. I usually go back and top off the tank after this.
Leave this in the tanks and lines for at least 4 hours of contact time. More is better here and if I'm doing this in the evening, I let it sit overnight. Then open the fresh tank drain, drain the tank, close the drain and refill it with water. Go back to the hot water faucet and run at least a coupla gallons more than the hot water tank holds through it. Then repeat the process with all the hot and cold faucets and spray hoses, running them longer this time to get as much of the bleach out as possible. Top the fresh tank off again.
Let this water sit an hour or so, then drain the fresh tank again, and close the drain.
When you fill the fresh tank this time, if you're sensitive to chlorine, dilute a pound of baking soda in a gallon of water for every 40 gallons of fresh tank capacity, pre-fill with this gallon, and then top off the tank.
Go back around and do the faucet thing again, starting with enough from a hot to flush the water heater, using plenty of water through each faucet. Let it sit an hour or so and drain the tank again.
Then do the final fill of the tank.
Works for me.
BTW, I usually pull the filter elements out and toss 'em when winterizing, and put new ones in after sanitizing the filters in spring.
I also read somewhere where an owner chains his drinking hoses together, pours the initial bleach/water mixture directly in them, and then hooks up the hoses to fill the tank. Sanitizing the hoses is probably a good idea that I haven't been doing.
And it's potable (drinkable) water without the r, even though it is portable.