I used a strong solution of bleach on my virgin tank that was full of water. Don't remember now, but maybe a quarter cup - probably whatever FWC recommended. Then like Cayuse let it slosh around for a few days and pumped it though sink and shower. Filled and drained a couple times.
Subsequent years I probably used less than a teaspoon for a half or 3/4 tank*. As a single camper I don't fill the tank all the way. I would add water as needed through the season, but no more bleach. I drain the tank at the end of the season before taking off and storing camper for winter. If I'm late I'll use a couple gallons RV antifreeze rather than risk frozen pipes. Come spring I flush the antifreeze a couple times, then repeat.
This worked great for many years, then one trip to Colorado I discovered my water had turned all funky - looked ok, but stank! Luckily I was near Silverton where I bought a small bottle of bleach. I put maybe a cup in my half full tank and drove on. By that night it was no longer funky, but very bleachy (if that is a word.) I got gallon jugs of water for drinking for the rest of the trip, but used the tank for hand washing only. Drained the tank when home and flushed several times. Just fine the rest of the season.
* Edited to note that I live in the country with non-chlorinated well water.