Not sure if you actually figured it out or not. The hose hookup that you found sounds to me like a grey or black tank flusher, not a city water hookup. Easy way to tell: hook up a hose and pressurize it. With no inside faucets open, there should be no water running. If you can hear water running, it is not the city water hookup, but rather a tank flusher.
The large unthreaded inlet that you have on the driver's side is to gravity fill the tank. It goes straight to the tank; nowhere else. Don't, under any circumstances, make an adapter to hook up a city water hose to that. Coming out of the bottom of the tank is a line that runs to the pump. Everything AFTER the pump is designed to operate under pressure, and that pressure can be supplied by either the pump or the city water system. Your tank is NEVER pressurized. NEVER. Did I say NEVER? Good.
Based on what your manual says, there is a city water hookup somewhere. That hookup will be connected directly to the pressurized side of the system. There is also, according to the manual, a valve somewhere in that initial line (fron the city water hookup) that allows you to divert city water into the tank, as an additional way to fill the tank. However, that must be monitored closely and turned off AS SOON AS THE TANK IS FULL or you WILL BURST the tank!!!