Assuming you have a similar water system to mine.
Here is what I do:
Step 1: open valve(grey plastic knob attached to water tank inside)
Step 2: hook a hose from your spigot to the city water outlet on the side of van
Step 3: turn on hose spigot
Step 4: there is an overflow tube on the water tank so when you see water dripping/flooding from the bottom of the van to the ground the tank is now full and you can turn off your spigot
Step 5: then close the water valve on the water tank
Step 6: disconnect hose from city water outlet
Here is a picture of my 10g water tank:
And here is the page from the SMB manual.