While you can use the overflow, you have to pry the cover off and likely will have future issues keeping it attached. The easiest way is to use the Fresh Water Tank Fill option in the Convenience Center. Set the valve to the Tank Fill, connect the male end of a water hose to the RV's input connection, pour your bleach into the water hose (use a funnel to make it easy), attach the female side of the hose to the water supply hose bib and turn on the water. The bleach will be pushed into the tank by the water which is filling it.
