What lanerd said. You should definitely do this before the beginning of each camping season.
This is not a case of more is better, so don't use more than the amount of bleach recommended. Otherwise you will have a hard time getting the bleach smell out of your system.
Even if you fill with chlorinated water, the chlorine breaks down over time. All kinds of algae, bacteria, and mold are introduced every time you hook up a supply hose. And the inside of the tank is not sterile to start with. The tank is not airtight and anything airborne will eventually get in.
These organisms may or may not be anything that would make you sick. But they can definitely produces a bad taste or smell, and even plug up your pump. And they could be things that make you sick. So sanitizing at least once a year, or maybe more, depending on use, is a very good idea.