x2 on what Beau said. You don't say which coach you have and that makes a difference. On most coaches if you turn on the pump it's pulling from the onboard fresh water tank, it won't act as a booster to the city water. What happens is the pump is adding to what is coming in from your hose. For this to work you'll have to have water in your onboard tank. We always keep at least half a tank of water for times just like that, plus to use on the road.
Always make sure you have a pressure regulator at the campground spigot to be safe, some water systems have low pressure at certain times of the day and high at others.
Hope this is helpful to you.
Dave & Myra
One day your life will flash before your eyes,
make sure it's worth watching!