About once a year when it's 100 degrees, and not using a lot of water we end up with an odor in the tank water. We clean it out with a tank of bleach added water, and it is normal for another year or so. I'm wondering if we should just add a bit of bleach to the system when it gets really hot. We don't drink the tank water, but do use it for dishes, showers etc. A pool supply said the bleach would be safer, as what they use for pools etc. is too strongly concentrated. There is also a product in the RV store, but not sure about the chemicals in it, so haven't tried it.
Has anyone else had this problem, and found a permanent solution?

Your doing the right thing. When the weather is hot, bleach or chlorine will dissipate and loose its sterilization efficiency. If there are any organisms in the tank, the warm water may cause them to grow.

Best is to use up all the water in the tank. Before refilling, add some Clorox. That should clear up the problem.

Don't overdue the bleach, since it could be damaging to any washers or, gaskets, etc.

You are on the right track. Also note that bleach you pick up at pool supply stores is 10% and bleach you buy at other markets is 5%. Same stuff but cost less to buy at pool supply shops. Mix pool bleach with 50% water and you had household bleach. I use it all the time for everything that needs to be cleaned up outside. Saves a little money. Also Camping World has some stuff for water tanks, high dollar and would work no better than bleach.

Even if you RV year-round, it's still a good idea to flush your water system with a weak bleach solution once a year. It doesn't take much bleach to sanitize the tank and lines. If you're not planning on using your RV for some time after flushing it with the bleach solution, flush it again with plain water.