fresh water tank care

Here's the standard (NFPA 1192-2011 Standards on RVs):

A.7.3.7.5 To ensure complete disinfection of the potable water
system, it is recommended that the following procedures
be followed on a new system, one that has not been used for a
period of time, or one that could have become contaminated.
This procedure is also recommended before long periods of
storage such as over winter.

(1) Prepare a chlorine solution using 1 gal. (3.8 L) of water
and 1/4 cup (60 ml) household bleach (sodium hypochlorite
solution). With tank empty, pour chlorine
solution into the tank. Use 1 gal. (3.8 L) solution for
each 15 gal (57 L) of tank capacity. This procedure will
result in a residual chlorine concentration of 50 ppm in
the water system. If a 100 ppm concentration is required,
as discussed in A.7.3.7.5(3), use 1/2 cup of
household bleach with 1 gal. of water to prepare the
chlorine solution. Use 1 gal. of the solution for each
15 gal. of tank capacity.

(2) Complete filling of tank with potable water. Open each
faucet and run the water until a distinct odor of chlorine
can be detected in the water discharged. Do not forget the
hot water taps.

(3) Allow the system to stand for at least 4 hours when disinfecting
with 50 ppm residual chlorine. If a shorter
time period is desired, a 100 ppm chlorine concentration
should be permitted to stand in the system for at
least 1 hour.

(4) Drain and flush with potable water.

1 tablespoon to 1 gallon is the Clorox recommendation. Flush it through all of the lines and let it sit for at least 2 minutes. There are 16 tablespoons in a cup. So if you have a 90 gallon fresh water tank you would need 90 tablespoons or 90/16=5 1/2 cups of bleach.
However, it is not necessary to fill the tank to the top. You could estimate that your tank if half full and add the appropriate ratio of bleach, run it through the lines and then drive the coach for a 1/2 hour or so allowing the bleach to splash all around in the tank. Drain it all, refill with water only, flush the lines and you are done.

Once filled, while bleach is in tank, I like to drive it so bleach water splashes around in tank and gets the top of the tank.

Use 1/4 cup bleach to every 15 gallons. Connect the hose, pour the bleach in the hose, and fill the tank. Run EVERY faucet--hot and cold--until you smell bleach coming out. Let stand for 24 hours. Drain the tank, fill with water, and flush the lines again.

Two comments on wildtiad's reply....depending on the size of your fresh water tank one cup of bleach may be enough or not. I use a quarter cup per 10 gallons. Others believe differently. Second, using or not the fresh water the tank should be sanitized at least yearly. Easiest place to do is an RV park on a city sewer line, not septic system. You'll love the freedom we boondockers have!

Yes. You should put a cup or so of Clorox in the tank, then fill it full, turn on your pump and pump water through all your faucets until you smell the bleach. Let it stand overnight then drain the water tank, refill it, flush out all the systems, repeat. This should be done once a year. It doesn't take a lot of bleach, as your killing bacteria not washing your shorts.