There are two water inlets on the camper. One inlet is to fill the fresh water tank. The other inlet says "city water" and that one bypasses the fresh water tank.
When hooked to city water, no need to run the water pump as the city water is already pressurized. When drawing water from the on board fresh water tank, the on board water pump provides pressure.
And be sure to bleed the air out of the water heater so it's full or water before activating the electric heating element or you will burn it out quickly....also before igniting the gas to heat the water.
The water heater has a 'pop off valve' similar to those on a home water heater.
Once the water system is pressurized (water in fresh water tank with pump turned on OR connected to city water) simply lift the lever on the pressure relief valve/pop off valve and the air will escape. Hold it open until a good solid flow of water comes out then release the lever and it will snap shut.
I will just add a note about pressure. Many city water sources are higher pressure than the RV plumbing is designed for. Always us a pressure regulator on the city water hookup. Ideally place the regulator at the water spigot to protect your hose as well. Available at any RV parts, Walmart, Bi-Mart or Amazon. Cheap insurance.