The reason to consider cleaning the tank is usually what we in the water
treatment biz call "T & O" or taste and odor. T & O is caused by what is IN
the water and can leave a residual in the tank even when drained. The
easiest, most effective remedy is to disinfect with chlorine followed by the
use of baking soda to rid the chlorine taste. A good procedure is as
follows:  Note, however, that while chlorine will take care of giardia
lamblia, it will NOT do anything for cryptosporidium parvum.

1. Drain tank and refill halfway with clean, fresh water.
2. Mix 1/3 cup of household bleach (which is about a 2% chlorine
    solution) in a container with a gallon or two of clean water and pour
    this into the tank.
3. Top off the tank with fresh water and drive around the block to mix
    up the whole banana. Then open the faucet until you smell chlorine
    in order to get solution into the line to the faucet.
4. Let water stand a few hours then drain the tank.
5. To remove the chlorine odor, mix 1/2 cup of baking soda with a
    gallon of water and pour this into the tank. Fill the tank with fresh
    water, and pump a quart of so of this solution through the faucet.
6. Do another trip around the block, drain the tank, refill with a couple
    gallons & and again pump a quart or so throughout the faucet and
    you are good to go. As has been said, it is best to keep the tank
    empty until you go on your next trip to protect against
    possible T & O.

Kit Carter, 85 Westy
Central Coast, CA (where it is STILL way too hot!)