The pressure regulator will protect your water system from an over pressure event to prevent damage. If the external water pressure falls below the regulated pressure, the regulator cannot maintain an even system pressure.

The regulator will limit a high pressure, but cannot maintain a set limit when the source is lower. It's the same result as when your gas bottles run empty. Once the system pressure falls below the regulated amount, the regulator no longer can control the pressure.

The OEM electric pump is self regulated, automatically kicking on at a low pressure and off when the desired ~45-60 PSI is reached.

Apparently the city water source was compromised. The hose to the trailer could have been kinked, stepped upon or other wise restricted. If many campers were using the water system at the same time, the supply line could have been of an insufficient volume to maintain the pressure. The inlet hose itself could have internally collapsed or been damaged.

Because your water system worked before, during and after this incident, leads me to believe the problem was the supply and not your LQ's system.

You can quickly test the system by hooking up your garden hose to the external supply port. If all is well, it should work again and you'll know the status of your system.