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.