never used the city water input, just fill the fresh tank.
1) we always leave home with full fresh water tank, that way we have good potable soft water at the campsite, and if the campsite has hard water use an inline water softener.
2) by using fresh tank I know that my grey/black tanks won't be near full till I go through over 3 fresh tank fills
3) when we leave the trailer, pump goes off, while if hooked to city water one would need to turn off the city water, less convienient. yes, that's anal but i do know a few campers who have had water leaks with city water even with a real pressure regulator (watts pressure regulator).
Black/grey tanks stay closed till full. unless you have a dip in the sewer hose if the valves are open you have no p trap in the campground sewer line, can lead to sewer gas and smell around the trailer. And it avoides a poop pyramid.