A few methods are possible. The first, as generally described by ol' yeller, is to pour bleach down the hose before filling the tank. I usually do this, though I tend to put the bleach down from the faucet end of the hose than the RV end of the hose...not that it makes a bit of difference really.
A second option, if you have a winterize setup that can pump antifreeze from the containers, is to mix up a big pot of water and bleach solution (with however much bleach you'll need), set the pump inlet for suction from the winterize port and open the tank fill valve, and pump the solution from the pot via the winterize hose into the water tank. It might be a reasonable idea to have water in the rest of the system to help force the solution into the tank vs. into the pipes.
Another option is to use various external hand or electric pumps to pump the solution into the city water connection. That seems like unnecessary effort and complication to me since the first and perhaps the second method are available and don't need other equipment.
