A city water has a threaded "swivel" fitting on the interior side. Often inside a cabinet or behind a bank of drawers. You could get access to the interior side of the inlet and punch out the check valve. They're just cheapo plastic gizmos. I removed ours and replaced it with a commercial grade brass one. Perhaps that would help you?
AFAIK, check valves inside city water inlets are an integral part and you'd need to replace the inlet. That would require caulking it. I'd opt for a separate new one inside.
Is the filter screen still in the city water inlet? They can sometimes plug up with silt & debris. Same thing can happen to the check valve.
A regulator is always a good idea. CG water pressure can get very high and I've heard of it getting as high as 200 psi. We've seen as high as 140 psi. If you travel around much, you won't know what kind of pressure you'll have. Also, sometimes pressure can be at an acceptable level but there can be a surge that could possibly cause damage.
