"City Water" is probably where you would hook the hose when using full hookups. there should be a check valve installed in it.
"Potable water" is probably the tank fill connection. This is where you want to put the bleach solution, to sanitize the tank and all the pipes. Let the solution sit several hours, preferable overnight, then drain, flush the tank and pipes, drain again, then fill with fresh water.
The unmarked connection is probably the black tank flush. Put water pressure to this ONLY with the black tank dump valve open!
