Actually the grey tubing is Polybutylene. It was installed in Mobile homes, travel trailers and houses from about 1978 to about 1995. There was a large class action suit due to problems with it breaking down and eventually leaking and causing water damage. You would be better off replacing all the grey tubing with PEX. As you can see the water heater tubing has already failed. And cpvc is actually a better choice over PVC for hot water use, but PEX is even better and a lot more flexible. It can be used with the correct support brackets to make short bends.