There are a few variants on how the water inlets are configured for RVs. If you have a single connection for both filling the tank and city water usage (and a fill valve), which I assume is the case, and a winterizing suction feed, you can indeed pump the sanitizing solution into the tank from a container by setting the valves for pumping antifreeze and opening the fill valve. (Leave the low point drains and faucet taps off for this, of course). Typically this will involve closing a valve between the pump inlet and the tank outlet, and opening one between the pump inlet and the connection for the winterize hose.
The winterize hose attachment is probably not the same connection as the main water inlet. Mine is inside a storage bay, while the city water connection is on the outside of the motorhome.
Pouring the bleach down the hose also works fine. Do remember to wear old clothes while sanitizing; don't ask me how I know that. :B