There are two basic systems for using "city water" to fill a fresh water holding tank.
My travel trailer uses a "gravity fill" system. Insert a hose from city water into the fill hole and run water in until it overflows.
Some RV's use a valve system in a "wet bay" to fill the holding tank. Some have both systems.
Connect a hose from the city water to the "city water" fitting. Set the holding tank fill valve to fill the tank. Shut the fill valve back to "off" when tank is full.
Disconnect the city water hose and turn the 12 volt water pump "on" to use the holding tank water.
Other than shutting off the fill valve as noted above, you do not usually need to switch a valve between city water and holding tank. You can usually leave the city water connected and turn the pump on at the same time. Water will flow from the highest presser source.
There is a check valve in the city water system to prevent pump water from flowing back into a lower presser city water system.
I wish you good luck and happy trails ahead!