Do you have a water pressure regulator on the city spigot? If so is it adjustable? I had that problem last month at a campground. Last used last year was at a campground with very low water pressure. I had the regulator adjusted to up the pressure a bit. First use out of the gate this year was at a park with good high pressure, actually park was almost empty, so very little demand, hence high pressure. I noticed my rig was starting to list to the right, bathroom door swinging, that was first clue. Re-leveled coach but later noticed it was noticeable listing right, what it looked like when I filled my fresh water tank with 100 gal of water for dry camping.
To my surprise when I looked at my fresh water tank it was almost full!! I only had approx 20 gal when we left home, just enough for traveling, washing hands, flushing toilet making ice etc. When I checked my adjustable regulator is was up over 70 psi. Yikes, I immediately lowered it to under 45 psi, where I normally have it, drained the tank back down to less than 25 gal and haven't a problem on the current trip we are on.
If that's not an issue then it may be a check valve that's failed!