The city water enters the pipes AFTER the RV water pump. City water will not fill the water tank. City water will pressurize all the pipes downstream from the water pump, which is downstream from the water tank. Water will not flow backwards through the water pump to fill the tank either. There might be a valve lever to turn to send pressurized water from the city connection to the water tank. You know when the tank is full as water flows out the fill port of the water tank. Turn valve off at this point.
Some may not have a by-pass valve at all, so the only way to put water in the water tank is to physically run the hose into the water tank fill port.
diagram in words:
Water tank > Water pump > pipes > faucets/water heater/shower/toilet.
City connection > pipes > faucets/water heater/shower/toilet.
Water fill port (on outside of RV) > water tank > water pump > pipes > faucets/water heater/shower/toilet.
if included:
Bypass valve > water tank, AND Water tank > fill port > outside world.