How to clean fresh water tank?

Use the bleach method and then put 1 or two ounces of hydrogen peroxide in and refill the tank. Every bit of the bleach will now be gone.
You can drink the water as-is or drain and refill with fresh again.

Ever since I actually looked inside my underbelly at my water tank, I feel complelled to tell folks what I found. I suspect many are in the same situation.

I have no data on percentages of how many water tanks are like this, but I suspect its a whole bunch of them. If you think your FW tank has the pump inlet and/or drain in the bottom of the tank, you may be surprised. Mine is in the side, down low in the corner. IOW, it can never be truly emptied! That is, unless you have the trailer tilted pretty steep to that corner. When my trailer is level, there is over an inch of water still remaining that cannot be drained. Keep this in mind when "sanitizing" or flushing the tank. I sometimes open the drain with the trailer tilted and just let it flush for a few minutes to get ALL the old water out. If you never do this, it's hard to say what might be down there. Obviously, if you dilute it enough and get the minimum concentration of bleach down there, it will kill any nasties that linger. Still it would be nice to be able to completely flush all the water out.

One more thing I don't understand why mfg'ers do the way they do.

Sandia Man wrote:
Most RV manufacturers suggest 1/4 cup for every 15 gallons for sanitizing, I do our TT when getting our rig ready for Spring camping.

What Sandia Man said!

Here is a thread I started last spring to remind everyone how to do this properly. It has step by step instructions.

Disinfect your water system

I drive around the block a few times after filling to water tank to mix the bleach/water and to slosh the solution onto the top of the tank to disinfect it too. Then I run the solution through all the plumbing.

Most RV manufacturers suggest 1/4 cup for every 15 gallons for sanitizing, I do our TT when getting our rig ready for Spring camping.

An old timer RVer told us to go out and buy the cheapest gallon of wine we could find and let it slosh around in the tank while traveling down the road. Then flush and fill with water. No bleach taste but may get a little wine flavor with the first tank of water.